THE TRANSFER OF THIS WARRANT IS RESTRICTED AS DESCRIBED HEREIN.

Exhibit 10.4

THE WARRANT REPRESENTED BY THIS CERTIFICATE AND THE SHARES ISSUABLE UPON EXERCISE HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”), OR ANY STATE SECURITIES LAWS AND NEITHER SUCH SECURITIES NOR ANY INTEREST THEREIN MAY BE OFFERED, SOLD, PLEDGED, ASSIGNED OR OTHERWISE TRANSFERRED UNLESS (1) A REGISTRATION STATEMENT WITH RESPECT THERETO IS EFFECTIVE UNDER THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS, OR (2) THE COMPANY RECEIVES AN OPINION OF COUNSEL TO THE HOLDER OF SUCH SECURITIES, WHICH COUNSEL AND OPINION ARE REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH SECURITIES MAY BE OFFERED, SOLD, PLEDGED, ASSIGNED OR TRANSFERRED IN THE MANNER CONTEMPLATED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT OR APPLICABLE STATE SECURITIES LAWS.

W-4122

THE TRANSFER OF THIS WARRANT IS

RESTRICTED AS DESCRIBED HEREIN.

BIOVEST INTERNATIONAL, INC.

Warrant for the Purchase of up to Two Million Shares

of Common Stock, par value $ 0.01 per share

Dated: May 9, 2008

THIS WARRANT CERTIFIES that, for value received, Ronald E. Osman (including any transferee, the “Holder”), is entitled to subscribe for and purchase from BIOVEST INTERNATIONAL, INC., a Delaware corporation (the “Company”), upon the terms and conditions set forth herein, up to an aggregate of Two Million (2,000,000) Shares of Common Stock, par value $ 0.01 per share (“Common Stock”), at a price (the “Exercise Price”) equal to $0.50 per share. All warrants are fully vested on the date hereof.

All rights to purchase shares pursuant to this Warrant must be exercised before 5:00 P.M. on May 8, 2015, New York time (the “Exercise Period”), As used herein the term “this Warrant” shall mean and include this Warrant and any Warrant or Warrants hereafter issued as a consequence of the exercise or transfer of this Warrant in whole or in part.

The number of shares of Common Stock issuable upon exercise of the Warrants (the “Warrant Shares”) and the Exercise Price may be adjusted from time to time as hereinafter set forth.

This Warrant may be exercised during the Exercise Period, as to the whole or any lesser number of whole Warrant Shares, by the surrender of this Warrant (with the “Election to Exercise” attached hereto, duly executed) to the Company at its office at 324 Hyde Park Avenue, Suite 350, Tampa FL 33606 or at such other place as is designated in writing by the Company, together with cash or a certified or bank cashiers check payable to the order of the Company in an amount equal to the Exercise Price multiplied by the number of Warrant Shares for which this Warrant is being exercised. Each Warrant not exercised prior to the expiration of the Exercise Period shall become null and void and all rights thereunder shall cease as of such time.

Notwithstanding any provisions herein to the contrary, if the Fair Market Value of one share of Common Stock is greater than the Exercise Price (at the date of calculation as set forth below), in lieu of exercising this Warrant for cash, the Holder may elect to receive shares equal to the value

(as determined below) of this Warrant (or the portion thereof being exercised) by surrender of this Warrant at the principal office of the Company together with the properly endorsed Exercise Notice in which event the Company shall issue to the Holder a number of shares of Common Stock computed using the following formula:

The Holder shall be deemed to be the holder of record of the Warrant Shares as soon as the Company receives the Warrant, the “Election to Exercise” and the aggregate Exercise Price for the Warrant Shares in accordance with Section 1; provided, however, that if the date of such receipt is a date upon which the transfer books of the Company are closed, the Holder shall be deemed to be the record holder on the next succeeding business day on which such books are open. As soon as practicable after each such exercise of this Warrant, the Company shall issue and cause to be delivered to the Holder a certificate or certificates for the Warrant Shares issuable upon such exercise, registered in the name of the Holder or its designee. If this Warrant is exercised in part only, the Company shall, upon surrender of this Warrant for cancellation, execute and deliver a new Warrant evidencing the right of the Holder to purchase the remaining unexercised balance of the Warrant Shares (or portions thereof) subject to purchase hereunder.

Any Warrants issued upon the transfer or exercise in part of this Warrant shall be numbered and shall be registered in a Warrant Register as they are issued. The Company shall be entitled to treat the registered holder of any Warrant on the Warrant Register as the owner in fact thereof for all purposes and shall not be bound to recognize any equitable or other claim to or interest in such warrant on the part of any other person, and shall not be liable for any registration or transfer of Warrants which are registered or to be registered in the name of a fiduciary or the nominee of a fiduciary unless made with the actual knowledge that a fiduciary or nominee is committing a breach of trust in requesting such registration or transfer, or with the knowledge of such facts that its participation therein amounts to bad faith. This Warrant shall be transferable only on the books of the Company upon delivery thereof duly endorsed by the Holder or by his duly authorized attorney or representative, or accompanied by proper evidence of succession, assignment, or authority to transfer. In all cases of transfer by an attorney, executor, administrator, guardian, or other legal representative, duly authenticated evidence of his or its authority shall be produced. Upon any registration of the transfer of this Warrant, the Company shall cause to be delivered a new Warrant or Warrants to the person entitled thereto. This Warrant may be exchanged, at the option of the Holder thereof, for another Warrant, or other Warrants of different denominations, of like tenor and representing in the aggregate the right to purchase a like number of Warrant Shares (or portions thereof), upon surrender to the Company or its duly authorized agent. Notwithstanding the foregoing, the Company shall have no obligation to cause Warrants to be transferred on its books to any person if, in the opinion of counsel to the Company, such transfer does not comply with the provisions of the Securities Act of 1933, as amended (the “Act”), and the rules and regulations thereunder.

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The Holder acknowledges that he has been advised by the Company that neither this Warrant nor the Warrant Shares have been registered under the Act, that this Warrant is being or has been issued and the Warrant Shares may be issued on the basis of the statutory exemption provided by Section 4(2) of the Act or Regulation D promulgated thereunder, or both, relating to transactions by an issuer not involving any public offering, and that the Company’s reliance thereon is based in part upon the representations to the Company made by the original Holder. The Holder acknowledges that he is familiar with the nature of the limitations imposed by the Act and the rules and regulations thereunder on the transfer of securities. In particular, the Holder agrees that no sale, assignment or transfer of this Warrant or the Warrant Shares issuable upon exercise hereof shall be valid or effective, and the Company shall not be required to give any effect to any such sale, assignment or transfer, unless (i) the sale, assignment or transfer of this Warrant or such Warrant Shares is registered under the Act, it being understood that neither this Warrant nor such Warrant Shares are currently registered for sale and that the Company has no obligation or intention to so register this Warrant or such Warrant Shares except as specifically provided herein, or (ii) this Warrant or such Warrant Shares are sold, assigned or transferred in accordance with all the requirements and limitations of Rule 144 under the Act, it being understood that Rule 144 is not available at the time of the original issuance of this Warrant for the sale of this Warrant or such Warrant Shares and that there can be no assurance that Rule 144 sales will be available at any subsequent time, or (iii) such sale, assignment, or transfer is otherwise exempt from registration under the Act.

In the event that the Company plans to file a registration statement with the U. S. Securities and Exchange Commission covering shares of common stock of the Company (“Registration Statement”), the Company 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. Notwithstanding the foregoing, the Company shall have full discretion to determine not to include the shares underlying the warrant in any registration statement if the Company reasonably determines that such registration may adversely effect the registration statement, the offering described in the registration statement or otherwise adversely effect the Company.

The Holder represents that he/she is an “Accredited Investor” as defined in the Act.

The Company shall at all times reserve and keep available out of its authorized and unissued Common Stock solely for the purpose of providing for the exercise of the rights to purchase Warrant Shares granted pursuant to the outstanding Warrants, such number of shares of Common Stock as shall, from time to time, be required therefor. The Company covenants that the Warrant Shares, upon receipt by the Company of the full Exercise Price therefor, shall be validly issued, fully paid, nonassessable, and free of preemptive rights.

In case of any consolidation with or merger of the Company with or into another corporation (other than a merger or consolidation in which the Company is the surviving or continuing corporation), or in case of any sale, lease, or conveyance to another corporation of the property and assets of any nature of the Company as an entirety or substantially as an entirety, such successor, leasing, or purchasing corporation, as the case may be, the Company shall (i) execute with the Holder an agreement providing that the Holder shall have the right thereafter to receive upon exercise of this Warrant solely the kind and amount of shares of stock and other securities, property, cash, or any combination thereof receivable upon such consolidation, merger, sale, lease, or conveyance by a holder of the number of shares of Common Stock for which this Warrant might have been exercised immediately prior to such

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consolidation, merger, sale, lease, or conveyance, and (ii) make effective provision in its certificate of incorporation or otherwise, if necessary, to effect such agreement. Such agreement shall provide for adjustments which shall be as nearly equivalent as practicable to the adjustments in Section 5.

In case of any reclassification or change of the shares of Common Stock issuable upon exercise of this Warrant (other than a change in par value or from no par value to a specified par value, or as a result of a subdivision or combination, but including any change in the shares into two or more classes or series of shares), or in case of any consolidation or merger of another corporation into the Company in which the Company is the continuing corporation and in which there is a reclassification or change (including a change to the right to receive cash or other property) of the shares of Common Stock (other than a change in par value, or from no par value to a specified par value, or as a result of a subdivision or combination, but including any change in the shares into two or more classes or series of shares), the Holder shall have the right thereafter to receive upon exercise of this Warrant solely the kind and amount of shares of stock and other securities, property, cash, or any combination thereof receivable upon such reclassification, change, consolidation, or merger by a holder of the number of shares of Common Stock for which this Warrant might have been exercised immediately prior to such reclassification, change, consolidation, or merger.

The issuance of any shares or other securities upon the exercise of this Warrant, and the delivery of certificates or other instruments representing such shares or other securities, shall be made without charge to the Holder for any tax or other charge in respect of such issuance. The Company shall not, however, be required to pay any tax which may be payable in respect of any transfer or delivery of this Warrant to a person other than, or the issuance and delivery of any certificate in a name other than that of the registered Holder and the Company shall not be required to issue or deliver any such certificate unless and until the person or persons requesting the issue thereof shall have paid to the Company the amount of such tax or shall have established to the satisfaction of the Company that such tax has been paid.

The Warrant Shares issued upon exercise of the Warrants shall be subject to a stop transfer order and the certificate or certificates evidencing such warrant Shares shall bear the following legend:

“THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”), OR ANY STATE SECURITIES LAWS AND NEITHER SUCH SECURITIES NOR ANY INTEREST THEREIN MAY BE OFFERED, SOLD, PLEDGED, ASSIGNED OR OTHERWISE TRANSFERRED UNLESS (1) A REGISTRATION STATEMENT WITH RESPECT THERETO IS EFFECTIVE UNDER THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS, OR (2) THE COMPANY RECEIVES AN OPINION OF COUNSEL TO THE HOLDER OF SUCH SECURITIES, WHICH COUNSEL AND OPINION ARE REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH SECURITIES MAY BE OFFERED, SOLD, PLEDGED, ASSIGNED OR TRANSFERRED IN THE MANNER CONTEMPLATED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT OR APPLICABLE STATE SECURITIES LAWS.”

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In addition, any Warrants issued upon transfer or any new Warrants issued shall bear a similar legend.

Upon receipt of evidence satisfactory to the Company of the loss, theft, destruction, or mutilation of any Warrant (and upon surrender of any Warrant if mutilated), including an affidavit of the Holder thereof that this Warrant has been lost, stolen, destroyed or mutilated, together with an indemnity against any claim that may be made against the Company on account of such lost, stolen, destroyed or mutilated Warrant, and upon reimbursement of the Company’s reasonable incidental expenses, the Company shall execute and deliver to the Holder thereof a new Warrant of like date, tenor, and denomination.

The Holder of any Warrant shall not have solely on account of such status, any rights of a stockholder of the Company, either at law or in equity, or to any notice of meetings of stockholders or of any other proceedings of the Company, except as provided in this warrant.

This Warrant shall be construed in accordance with the laws of the State of Florida applicable to contracts made and performed within such State, without regard to principles governing conflicts of law.

The Company irrevocably consents to the jurisdiction of the courts of the State of Florida and of any federal court located in such State in connection with any action or proceeding arising out of or relating to this Warrant, any document or instrument delivered pursuant to, in connection with or simultaneously with this Warrant, or a breach of this Warrant or any such document or instrument.

Any notice or other communication required or permitted to be given hereunder shall be in writing and shall be mailed by certified mail, return receipt requested, or by Federal Express, Express Mail or similar overnight delivery or courier service or delivered (in person or by telecopy, telex or similar telecommunications equipment) against receipt to the party to whom it is to be given, (i) if to the Company, at 324 Hyde Park Avenue, Suite 350, Tampa FL 33606, Attention: President, (ii) if to the Holder, at its address set forth on the first page hereof, or (iii) in either case, to such other address as the party shall have furnished in writing in accordance with the provisions of this Section 13. Notice to the estate of any party shall be sufficient if addressed to the party as provided in this Section 13. Any notice or other communication given by certified mail shall be deemed given at the time of certification thereof, except for a notice changing a party’s address which shall be deemed given at the time of receipt thereof. Any notice given by other means permitted by this Section 13 shall be deemed given at the time of receipt thereof.

This Warrant may be amended only by a written instrument executed by the Company and the Holder hereof. Any amendment shall be endorsed upon this Warrant, and all future Holders shall be bound thereby.

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FORM OF ASSIGNMENT

(To be executed by the registered holder if such holder desires to transfer the attached Warrant.)

FOR VALUE RECEIVED,                                          hereby sells, assigns, and transfers unto                                          a Warrant to purchase                      shares of Common Stock, par value $0.01 per share, of BIOVEST INTERNATIONAL, INC. (the “Company”), together with all right, title, and interest therein, and does hereby irrevocably constitute and appoint                                          attorney to transfer such Warrant on the books of the Company, with full power of substitution.

NOTICE

The signature on the foregoing Assignment must correspond to the name as written upon the face of this Warrant in every particular, without alteration or enlargement or any change whatsoever.

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ELECTION TO EXERCISE

The undersigned hereby exercises his or its rights to purchase                          Warrant Shares covered by the within Warrant and tenders payment herewith in the amount of $             in accordance with the terms thereof, and requests that certificates for such securities be issued in the name of, and delivered to:

(Print Name, Address and Social Security or Tax Identification Number)

and, if such number of Warrant shares shall not be all the Warrant Shares covered by the within Warrant, that a new Warrant for the balance of the Warrant Shares covered by the within Warrant be registered in the name of, and delivered to, the undersigned at the address stated below.

(Print Name, Address and Social Security or Tax Identification Number)

and, if such number of Warrant Shares shall not be all the Warrant Shares covered by the within Warrant, that a new Warrant for the balance of the Warrant Shares covered by the within Warrant be registered in the name of, and delivered to, the undersigned at the address stated below.

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