WARRANT

Exhibit 10.13

WARRANT

Dated as of April 25, 2006

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR THE SECURITIES LAW OF ANY STATE AND MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER SUCH ACT AND APPLICABLE STATE SECURITIES LAWS OR PURSUANT TO AN APPLICABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF SUCH ACT AND SUCH LAWS.

BIOVEST INTERNATIONAL, INC.

COMMON STOCK PURCHASE WARRANT

Void after April 24, 2015

BIOVEST INTERNATIONAL, INC. (the “Company”), a Delaware corporation, hereby certifies that for value received, Telesis CDE Corporation or its successors or assigns (the “Holder”), is entitled, subject to Vesting, to purchase, subject to the terms and conditions hereinafter set forth, at any time or from time to time (the “Exercise Date”) and ending prior to 5:00 P.M., New York City time, on April 24, 2015 (the “Expiration Date”) up to 1,200,000 shares of Common Stock at an exercise price per share of $1.30 per share subject to adjustment as provided herein (the “Purchase Price”).

This Warrant is issued in connection with a Loan Agreement dated April 25, 2006 between the Company and, inter alia, Telesis CDE Two, LLC. The Holder agrees to duly execute and deliver the Investment Representation Statement attached hereto as Exhibit A (the “Investment Representation Statement”) to the Company on or before the date hereof and acknowledges that the Company is issuing this Warrant in reliance on the representations set forth in the Investment Representation Statement.

1. Definitions. For the purposes of this Warrant, the following terms shall have the meanings indicated:

“Black-out Period” shall mean a six month period commencing on the date that Biovest notifies Holder that it is seeking to raise financing pursuant to a public offering or a private offering requiring registration rights to be granted by Biovest to the purchaser. Holder agrees not to sell a under Rule 144 or otherwise any shares of Biovest stock underlying the Warrant during a Black-Out Period. The Black-out Period shall not apply to Holder during any Black-out Period after the date on which Accentia sells any shares of Biovest common stock held of record by Accentia at the begining of said Black-out Period.

“Business Day” shall mean any day other than a Saturday, Sunday or other day on which commercial banks in the City of New York are authorized or required by law or executive order to close.

“Closing Price” shall mean, with respect to each share of Common Stock for any day, (a) the last reported sale price regular way or, in case no such sale takes place on such day, the average of the closing bid and asked prices regular way, in either case as reported on the principal national securities exchange on which the Common Stock is listed or admitted for trading or (b) if the Common Stock is not listed or admitted for trading on any national securities exchange, the last reported sale price or, in case no such sale takes place on such day, the average of the highest reported bid and the lowest reported asked quotation for the Common Stock, in either case as reported on the OTCBB or a similar service if OTCBB is no longer reporting such information.

“Common Stock” means the common stock, no par value, of the Company, and any class of stock resulting from successive changes or reclassification of such Common Stock.

“Company” has the meaning ascribed to such term in the first paragraph of this Warrant.

“Current Market Price” shall be determined in accordance with Subsection 3(b).

“Exercise Date” has the meaning ascribed to such term in Subsection 2(c).

“Expiration Date” has the meaning ascribed to such term in the first paragraph of this Warrant.

“Issued Warrant Shares” means any shares of Common Stock issued upon exercise of the Warrant.

“NASDAQ” shall mean the Automatic Quotation System of the National Association of Securities Dealers, Inc.

OTCBB shall mean the Over the Counter Bulletin Board operated by the National Association of Securities Dealers.

“Person” shall mean any individual, firm, corporation, limited liability company, partnership, trust, incorporated or unincorporated association, joint venture, joint stock company, government (or an agency or political subdivision thereof) or other entity of any kind, and shall include any successor (by merger or otherwise) of such entity.

“Piggy-Back Registration Right” shall mean the right to require Biovest International, Inc. to include the proportionate number of shares of the common stock underlying the Warrant to be covered by a Registration Statement filed by Biovest International, Inc. with the U.S. Securities and Exchange Commission as part of a registration statement covering shares of Biovest International, Inc owned of record by Accentia Biopharmaceuticals, Inc. The phrase the “proportionate number of the shares of common stock underlying the Warrant” shall mean the greater of: (i) 200,000 shares of Biovest common stock or (ii) the number of shares underlying the Warrant on the calculation date divided by the number of shares of Biovest common stock owned by Accentia. To the extent that Accentia has engaged an underwriter to underwrite or place its shares of Biovest (“Accentia Underwriting”) in connection with the registration of shares held by Accentia, Holder shall be permitted to participate in such Accentia Underwriting to the extent permitted by the underwriter and in such instance Holder shall pay all the proportionate share of all costs and expenses related to the underwriting and registration statement and all commissions, discounts and other underwriting or placement costs or expenses related to Holder’s shares. The Piggy-Back Registration Right shall not give the Holder of the Warrant the right to require the shares underlying the Warrant to be covered by any registration statement filed by Biovest International, Inc. covering any shares other than share of common stock owned by Accentia Biopharmaceuticals, Inc.

“Purchase Price” has the meaning ascribed to such term in the first paragraph of this Warrant.

“Vesting” or “Vested” shall mean the date on which the Holder can purchase shares pursuant to the Warrant. The Warrant shall be Vested as follows: (i) 300,000 shares shall vest on the first day of the third year following the date hereof and 300,000 shares shall vest on each anniversary thereof until all shares are fully vested; (iii) all shares underlying the Warrant shall vest on the date on which Biovest sells all or substantially all of its assets or (iv) the number of shares which Holder is entitled to include in a Piggy-Back Registration Statement under the Piggy-Back Registration Rights granted hereunder shall vest on the effective date of such Piggy–Back Registration Statement.

“Warrant” shall mean this Warrant and any subsequent Warrant issued pursuant to the terms of this Warrant.

“Warrant Register” has the meaning ascribed to such term in Subsection 6(c).

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2. Exercise of Warrant

(a) Exercise. Once Vested, this Warrant may be exercised, in whole or in part, at any time or from time to time and ending on the Expiration Date, by surrendering to the Company at its principal office this Warrant, with the form of Election to Purchase Shares (the “Election to Purchase Shares”) attached hereto as Exhibit B and an updated Investment Representation Statement attached hereto as Exhibit A duly executed by the Holder and accompanied by payment of the Purchase Price for the number of shares of Common Stock specified in such form. or by surrendering this Warrant in exchange for the number of shares of Common Stock equal to the product of (x) the number of shares of Common Stock as to which this Warrant is being exercised, multiplied (y) by the Purchase Price, subject to adjustment as provided herein.

(b) Partial Exercise. If this Warrant is exercised for less than all of the shares of Common Stock purchasable under this Warrant, the Company shall cancel this Warrant upon surrender hereof and shall execute and deliver to the Holder a new Warrant of like tenor for the balance of the shares of Common Stock purchasable hereunder. This Warrant may be exercised in partial amounts of no less than 50,000 shares.

(c) When Exercise Effective. The exercise of this Warrant shall be deemed to have been effective immediately prior to the close of business on the Business Day on which this Warrant is surrendered to and the Purchase Price is received by the Company as provided in this Section 2 (the “Exercise Date”) and the Person in whose name any certificate for shares of Common Stock shall be issuable upon such exercise, as provided in Subsection 2(b), shall be deemed to be the record holder of such shares of Common Stock for all purposes on the Exercise Date.

3. Adjustment of Purchase Price and Number of Shares. The Purchase Price and the number of shares of Common Stock issuable upon exercise of this Warrant shall be adjusted from time to time upon the occurrence of the following events:

(a) Dividend, Subdivision, Combination or Reclassification of Common Stock. If the Company shall, at any time or from time to time, (i) declare a dividend on the Common Stock payable in shares of its capital stock (including Common Stock), (ii) subdivide the outstanding Common Stock into a larger number of shares of Common Stock, (iii) combine the outstanding Common Stock into a smaller number of shares of its Common Stock, or (iv) issue any shares of its capital stock in a reclassification of the Common Stock (including any such reclassification in connection with a consolidation or merger in which the Company is the continuing corporation), then in each such case, the Purchase Price in effect at the time of the record date for such dividend or of the effective date of such subdivision, combination or reclassification, and the number and kind of shares of capital stock issuable on such date shall be proportionately adjusted so that the Holder of any Warrant exercised after such date shall be entitled to receive, upon payment of the same aggregate amount as would have been payable before such date, the aggregate number and kind of shares of capital stock which, if such Warrant had been exercised immediately

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prior to such date, such Holder would have owned upon such exercise and been entitled to receive by virtue of such dividend, subdivision, combination or reclassification. Any such adjustment shall become effective immediately after the record date of such dividend or the effective date of such subdivision, combination or reclassification. Such adjustment shall be made successively whenever any event listed above shall occur. If a dividend is declared and such dividend is not paid, the Purchase Price shall again be adjusted to be the Purchase Price in effect immediately prior to such record date (giving effect to all adjustments that otherwise would be required to be made pursuant to this Section 3 from and after such record date).

(b) Determination of Current Market Price. The Current Market Price per share of Common Stock on any date shall be deemed to be the Closing Price per share of Common Stock on the day immediately preceding the date of determination. If on any such date the shares of Common Stock are not listed or admitted for trading on any national securities exchange or quoted by OTCBB or a similar service, then the Current Market Price shall be determined in good faith by the Board of Directors of the Company.

(c) De Minimis Adjustments. No adjustment in the Purchase Price shall be made if the amount of such adjustment would result in a change in the Purchase Price per share of less than $0.01, but in such case any adjustment that would otherwise be required to be made shall be carried forward and shall be made at the time of and together with the next subsequent adjustment, which together with any adjustment so carried forward, would result in a change in the Purchase Price of $0.01 per share or more.

(d) Adjustment of Number of Shares Issuable Upon Exercise. Upon each adjustment of the Purchase Price as a result of the calculations made in Subsection 3(a) this Warrant shall thereafter evidence the right to receive, at the adjusted Purchase Price, that number of shares of Common Stock (calculated to the nearest one-hundredth) obtained by dividing (x) the product of the aggregate number of shares of Common Stock covered by this Warrant immediately prior to such adjustment and the Purchase Price in effect immediately prior to such adjustment of the Purchase Price by (y) the Purchase Price in effect immediately after such adjustment of the Purchase Price.

(e) Reorganization, Reclassification, Merger and Sale of Assets. If there occurs any capital reorganization or any reclassification of the Common Stock of the Company, the consolidation or merger of the Company with or into another Person (other than a merger or consolidation of the Company in which the Company is the continuing corporation and which does not result in any reclassification or change of outstanding shares of its Common Stock) or the sale or conveyance of all or substantially all of the assets of the Company to another Person, then the Holder will thereafter be entitled to receive, upon the exercise of this Warrant in accordance with the terms hereof, the same kind and amounts of securities (including shares of stock) or other assets, or both, which were issuable or distributable to the holders of outstanding Common Stock of the Company upon such reorganization, reclassification, consolidation, merger, sale or conveyance, in respect of that number of shares of Common Stock then deliverable upon the exercise of this Warrant if this Warrant had been exercised immediately prior to such reorganization,

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reclassification, consolidation, merger, sale or conveyance; and, in any such case, appropriate adjustments (as determined in good faith by the Board of Directors of the Company) shall be made to assure that the provisions hereof (including provisions with respect to changes in, and other adjustments of, the Purchase Price) shall thereafter be applicable, as nearly as reasonably may be practicable, in relation to any securities or other assets thereafter deliverable upon exercise of this Warrant.

4. Fractional Shares. Notwithstanding an adjustment pursuant to Section 3(d) in the number of shares of Common Stock covered by this Warrant or any other provision of this Warrant, the Company shall not be required to issue fractions of shares upon exercise of this Warrant or to distribute certificates which evidence fractional shares. In lieu of fractional shares, the Company may make payment to the Holder, at the time of exercise of this Warrant as herein provided, of an amount in cash equal to such fraction multiplied by the Current Market Price of a share of Common Stock on the Exercise Date.

5. Replacement of Warrants. On receipt by the Company of an affidavit of an authorized representative of the Holder stating the circumstances of the loss, theft, destruction or mutilation of this Warrant (and in the case of any such mutilation, on surrender and cancellation of such Warrant), the Company at its expense will promptly execute and deliver, in lieu thereof, a new Warrant of like tenor which shall be exercisable for a like number of shares of Common Stock. If required by the Company, such Holder must provide an indemnity bond or other indemnity sufficient in the judgment of the Company to protect the Company from any loss which it may suffer if a lost, stolen or destroyed Warrant is replaced.

6. Restrictions on Transfer.

(a) The Holder acknowledges that the Warrant and the Common Stock issuable upon the exercise of the Warrant has not been registered under the Securities Act and may be transferred only pursuant to an effective registration under the Securities Act or pursuant to an applicable exemption from the registration requirements of the Securities Act. The Holder further acknowledges that the certificates representing the Issued Warrant Shares shall bear the following legend:

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”), AND MAY NOT BE OFFERED, SOLD, PLEDGED, HYPOTHECATED, ASSIGNED OR TRANSFERRED EXCEPT PURSUANT TO (I) AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT, (II) TO THE EXTENT APPLICABLE, RULE 144 UNDER THE ACT, OR (III) AN OPINION OF COUNSEL, IF SUCH OPINION SHALL BE REASONABLY SATISFACTORY TO COUNSEL TO THE ISSUER, THAT AN EXEMPTION FROM REGISTRATION UNDER THE ACT IS AVAILABLE.

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(b) With respect to a transfer that should occur prior to the time that the Warrant or the Common Stock issuable upon the exercise thereof is registered under the Securities Act or transferable pursuant to Rule 144 of the Act, such Holder shall request an opinion of counsel (which shall be rendered by counsel reasonably acceptable to the Company) that the proposed transfer may be effected without registration or qualification under any Federal or state securities or blue sky law.

(c) The Company shall maintain a register (the “Warrant Register”) in its principal office for the purpose of registering the Warrant and any transfer thereof, which register shall reflect and identify, at all times, the ownership of any interest in the Warrant. Upon the issuance of this Warrant, the Company shall record the name of the initial purchaser of this Warrant in the Warrant Register as the first Holder. Upon surrender for registration of transfer or exchange of this Warrant together with a properly executed Form of Assignment attached hereto as Exhibit C at the principal office of the Company, the Company shall, at its expense, execute and deliver one or more new Warrants of like tenor which shall be exercisable for a like aggregate number of shares of Common Stock, registered in the name of the Holder or a transferee or transferees.

7. Piggy-Back Registration Rights. In the event that Biovest International, Inc. plans to file a registration statement with the U. S. Securities and Exchange Commission covering shares of common stock of Biovest (“Registration Statement”), Biovest shall provide written notice to Holder and Holder shall have 30 days to require in writing that all shares of common stock underlying the Warrant, to the extent vested, be covered in the Registration Statement.

8. No Rights or Liability as a Stockholder. This Warrant does not entitle the Holder hereof to any voting rights or other rights as a stockholder of the Company. No provisions hereof, in the absence of affirmative action by the Holder hereof to purchase Common Stock, and no enumeration herein of the rights or privileges of the Holder shall give rise to any liability of such Holder as a stockholder of the Company.

9. Amendment or Waiver. This Warrant and any term hereof may be amended, waived, discharged or terminated only by and with the written consent of the Company and the Holder.

10. Notices. Any notice or other communication (or delivery) required or permitted hereunder shall be made in writing and shall be by registered mail, return receipt requested, telecopier, courier service or personal delivery to the Company at its principal office and to the Holder at its address as it appears in the Warrant Register. All such notices and communications (and deliveries) shall be deemed to have been duly given: when delivered by hand, if personally delivered; when delivered by courier, if delivered by commercial overnight courier service; five Business Days after being deposited in the mail, postage prepaid, if mailed; and when receipt is acknowledged, if telecopied.

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11. Governing Law. This Warrant shall be governed by and construed in accordance with the laws of the State of New York, without regard to the principles of conflicts of law of such State.

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12. Headings. The headings in this Warrant are for convenience of reference only and shall not limit or otherwise affect the meaning hereof.

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INVESTMENT REPRESENTATION STATEMENT

Shares of Common Stock of

BIOVEST INTERNATIONAL, INC.

In connection with the purchase through the exercise of a warrant of the above-listed shares of Common Stock (the “Securities”), the undersigned hereby represents to Biovest International, Inc. (the “Company”) as follows:

The Securities will be acquired for investment for its own account, not as a nominee or agent, and not with a view to the sale or distribution of any part thereof, and the undersigned has no present intention of selling, granting participation in or otherwise distributing the same in violation of the Securities Act of 1933, as amended (the “Act”), but subject, nevertheless, to any requirement of law that the disposition of its property shall at all times be within its control. By executing this Statement, the undersigned further represents that it does not have any contract, undertaking, agreement or arrangement with any person to sell, transfer, or grant participations to such person or to any third person in violation of the Act, with respect to any Securities issuable upon exercise of the Warrant.

The undersigned understands that the Securities issuable upon exercise of the Warrant at the time of issuance may not be registered under the Act, and applicable state securities laws, on the ground that the issuance of such securities is exempt pursuant to Section 4(2) of the Act and/or Regulation D - - Rule 506 thereunder and state law exemptions relating to offers and sales not by means of a public offering, and that the Company’s reliance on such exemptions is predicated on the undersigned’s representations set forth herein.

The undersigned recognizes that an investment in the Company involves substantial risks, too numerous and diverse to be adequately described, summarized, or listed herein. It is aware of and understands the nature and potential for such risks in an investment of this kind. It acknowledges receipt of or access to all reports filed by the Company under the Securities Act and the Exchange Act, including pursuant to Section 13(a) or 15(d) thereof, for the twelve months preceding the date hereof (or such shorter period as the Company was required by law to file such reports) (the foregoing materials, being collectively referred to herein as the “SEC Reports”). The undersigned has determined that the entering into this transaction is consistent with its investment objectives and that it is able to bear the substantial economic risks of losing its investment in the Company. Among other factors and based on present circumstances it has taken into consideration, it can afford to hold the Securities for an indefinite period and can afford a complete loss of its investment It understands that no governmental authority has passed on or made any recommendation or endorsement of the Securities. The undersigned understands that (A) the Securities have not been registered under the Securities Act or applicable state securities laws, and may be offered and sold under an exemption from registration provided by such laws and the rules and regulations thereunder; (B) it must bear the economic risks of the Securities, because the Securities cannot be resold unless subsequently registered under applicable securities laws or unless an exemption from such registration is available; and (C) the exemption provided by Rule 144 under the Securities Act may not be available because of the conditions and limitations of that rule, in the absence of the availability of that rule any disposition by it of any or all of the Securities may require compliance with some other exemption under the

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Securities Act, and the Company is not under any obligation and does not plan to take any action in furtherance of making that rule or any other exemption so available. It has been informed that legends referring to the restrictions indicated herein will be placed on documents evidencing or representing the Shares.

No representations or promises have been made concerning the marketability or value of the Securities.

The representations and warranties made by the undersigned herein are made by the undersigned with the intent that they be relied upon by the Company in determining the compliance of issuance of the shares with exemptions from federal or state securities laws.

The following information is needed to determine whether issuance of the Securities by the Company is legally permitted.

The undersigned understands that this questionnaire is to enable the Company to discharge its responsibilities under federal and state securities laws and that the Company will rely on the information contained herein. Accordingly, the undersigned represents and warrants to the Company that:

(a) the information contained herein is complete and accurate and may be relied on by the Company; and

The undersigned is an “Accredited Investor” within the meaning of Rule 501 of Regulation D of the Act, as presently in effect, because the undersigned meets one or more of the following criteria (please check where appropriate):

¨ (i) a natural person with individual net worth, or joint net worth with his/her spouse which presently exceeds $1,000,000;

¨ (ii) a natural person with individual income in excess of $200,000 in each of the two most recent years or joint income with his/her spouse in excess of $300,000 in each of those years and who has a reasonable expectation of reaching the same income level in the current year;

The undersigned agrees that in no event will it make a disposition of the Securities unless and until the Securities are registered under the Act or sold pursuant to Rule 144, or (i) it shall have notified the Company of the proposed disposition and shall have furnished the Company with a statement of the circumstances surrounding the proposed disposition, and (ii) it shall have furnished the Company with an opinion of counsel satisfactory to the Company and Company’s counsel to the effect that (A) appropriate action necessary for compliance with the Act and any applicable state securities laws has been taken or an exemption from the registration requirements of the Act and such laws is available, and (B) the proposed transfer will not violate any of said laws.

The undersigned acknowledges that the Securities must be held indefinitely unless subsequently registered under the Act or an exemption from such registration is available. The undersigned is aware of the provisions of Rule 144 promulgated under the Act which permit limited resale of shares purchased in a private placement subject to the satisfaction of certain conditions, including, among other things, the existence of a public market for the shares, the availability of certain current public information about the Company, the resale occurring not less than one year after a party has purchased and paid for the security to be sold, the sale being through a “broker’s

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transaction” or in transactions directly with a “market makers” (as provided by Rule 144(f)) and the number of shares being sold during any three-month period not exceeding specified limitations.

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[FORM OF]

ELECTION TO PURCHASE SHARES

The undersigned hereby irrevocably elects to exercise the Warrant to purchase              shares of Common Stock, no par value (“Common Stock”), of BIOVEST INTERNATIONAL, INC. (the “Company”) and hereby makes payment of $             therefor. The undersigned hereby requests that certificates for such shares be issued and delivered as follows:

(NAME)

(ADDRESS, INCLUDING ZIP CODE)

(SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER)

(NAME)

(ADDRESS, INCLUDING ZIP CODE)

If the number of shares of Common Stock purchased hereby is less than the number of shares of Common Stock covered by the Warrant, the undersigned requests that a new Warrant representing the number of shares of Common Stock not purchased be issued and delivered as follows:

(NAME OF HOLDER)

(ADDRESS, INCLUDING ZIP CODE)

(NAME OF HOLDER)

(ADDRESS, INCLUDING ZIP CODE)

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[FORM OF] ASSIGNMENT

FOR VALUE RECEIVED, the undersigned hereby sells, assigns, and transfers unto the Assignee(s) named below all of the rights of the undersigned to purchase Common Stock, no par value (“Common Stock”), of BIOVEST INTERNATIONAL, INC. represented by the Warrant, with respect to the number of shares of Common Stock set forth below:

and does hereby irrevocably constitute and appoint                              Attorney to make such transfer on the books of BIOVEST INTERNATIONAL, INC. maintained for that purpose, with full power of substitution in the premises.

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