WARRANT

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.

2

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