WARRANT

Exhibit 10.28

WARRANT

Dated as of December 14, 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 December 13, 2016

BIOVEST INTERNATIONAL, INC. (the “Company”), a Delaware corporation, hereby certifies that for value received, Dennis L. Ryll or his successors or assigns (the “Holder”), is entitled to purchase, subject to the terms and conditions hereinafter set forth, at any time or from time to time beginning on December 14, 2006, (the “Exercise Date”) and ending prior to 5:00 P.M., New York City time, on December 13, 2016 (the “Expiration Date”) up to 590,909 shares of Common Stock at an exercise price per share of $1.00 per share subject to adjustment as provided herein (the “Purchase Price”).

This Warrant is issued in connection with a Guaranty dated December 8, 2006 between the Holder and, inter alia, U.S. Bank.

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

“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 NASDAQ or a similar service if NASDAQ is no longer reporting such information.

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“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.

“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.

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

“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).

2. Exercise of Warrant

(a) Exercise. This Warrant may be exercised, in whole or in part, at any time or from time to time during the period beginning on December 14, 2006 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 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 by (y) a fraction, the numerator of which is: (1) the Current Market Price per share of Common Stock on the Exercise Date less the Purchase Price per share of Common Stock and the denominator of which is (2) the Current Market Price per share of Common Stock.

(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.

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(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 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 NASDAQ 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 5%, 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 5% 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.

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(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, 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,

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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.

(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, 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 B 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. 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.

8. 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.

9. 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.

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

11. 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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[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:

ISSUE TO:                                                                                                                                                                                                                                                

(NAME)

(ADDRESS, INCLUDING ZIP CODE)

(SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER)

DELIVER TO:                                                                                                                                                                                                                                         

(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:

ISSUE TO:                                                                                                                                                                                                                                                

(NAME OF HOLDER)

(ADDRESS, INCLUDING ZIP CODE)

DELIVER TO:                                                                                                                                                                                                                                         

(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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