Tax Credit Reimbursement And Indemnity Agreement, Dated As of April 25, 2006

Exhibit 10.15

GUARANTY

THIS GUARANTY (this " Guaranty" ) is made as of the 25 th day of April, 2006 by and among FRANCIS E. O' DONNELL, JR. (" O' Donnell" ), KATHLEEN M. O' DONNELL, TRUSTEE OF THE FRANCIS E. O' DONNELL, JR. IRREVOCABLE TRUST (the " O' Donnell Trust" ), DENNIS L. RYLL (" Ryll" ), RONALD OSMAN (" Osman" ), STEVEN J. STOGEL (" Stogel" ), DONALD L. FURGERSON (" Furgerson" ) and DONALD L. FURGERSON, TRUSTEE OF THE DONALD L. FURGERSON REVOCABLE TRUST (the " Furgerson Trust" ) ( O' Donnell, the O' Donnell Trust, Ryll, Osman, Stogel, Furgerson and the Furgerson Trust, whether one or more, hereinafter called " I ndividual Guarantor" in the singular), BIOVEST INTERNATIONAL, INC., a Delaware corporation (" Biovest" ), ACCENTIA BIOPHARMACEUTICALS, INC . (" Accentia" ) (Biovest and the Individual Guarantors, whether one or more, hereinafter called " Guarantor" in the singular) to and for the benefit of U.S. BANCORP COMMUNITY INVESTMENT CORPORATION, a Delaware corporation (" USB" ), and with respect to the guaranty set forth in Section 2B hereof, TELESIS CDE TWO, LLC, a Delaware limited liability company (the " CDE" ), and its managing member, TELESIS CDE CORPORATION , a Delaware corporation (" Telesis" ).

RECITALS WHEREAS, the CDE has received a sub-allocation of New Markets Tax Credits (the " Tax Credits" ) under Section 45D of the Internal Revenue Code of 1986, as amended, and the rules an regulations promulgated thereunder (collectively, the " Code" ).

WHEREAS, Biovax Investment, LLC, a Delaware limited liability company (the " Fund" ) has contributed equity to the CDE (the " QEI Contribution" ), which equity is expected to constitute a " qualified equity investment" (" QEI" ) under the New Markets Tax Credit program authorized by Section 45D of the Code (the " NMTC Program" ). WHEREAS, the QEI Contribution is being funded in part with the proceeds of equity contributed to the Fund by USB. The proceeds of the QEI Contribution will be used by the CDE to fund a loan to Biovax, Inc., a Florida corporation (" Borrower" ), in the original principal amount of $11,500,000 (the " CDE Loan" ), which is expected to constitute a " qualified low-income community investment" under the NMTC Program.

WHEREAS, the Tax Credits claimable by USB in connection with the QEI Contribution have allowed USB to provide the QEI Contribution to the CDE on more favorable terms, which in turn has allowed the CDE to provide the CDE Loan to Borrower on more favorable terms and, as a result, Borrower believes that it shall substantially benefit, directly or indirectly, from the making of the QEI Contribution.

WHEREAS, the Borrower is primarily engaged in the business of manufacturing an anti-cancer vaccine currently in clinical trial within a United States population census tract number which constitutes a Low-Income Community under the NMTC Program, and the proceeds of the CDE Loan will be used to finance certain activities of Borrower associated with the foregoing activities.

WHEREAS, in connection with the foregoing and concurrently herewith, USB and Borrower have entered into that certain Tax Credit Reimbursement and Indemnity Agreement (the " Indemnity" ).

WHEREAS, each Guarantor is an owner (directly or indirectly) of Borrower, USB' s acquisition of a membership interest in the Fund is of material benefit to each Guarantor, and without this Guaranty, USB would not acquire a membership interest in, or make its contribution to, the Fund. WHEREAS, to induce USB to acquire a membership interest in, and make its equity contribution to, the Fund, each Guarantor has agreed to enter into this Guaranty.

NOW, THEREFORE, in consideration of the foregoing, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto, intending to be legally bound hereby, each hereby covenants and agrees to and for the benefit of USB as follows:

1. Notwithstanding anything to the contrary set forth herein, as used herein, the terms " several" , " severally" , " joint and several" or " jointly and severally" as they relate to the Guaranteed Obligations hereunder mean:

(A) The obligation of O' Donnell and the O' Donnell Trust, collectively, as to any liability covered hereby shall be several and not joint and several with the liability of O' Donnell and the O' Donnell Trust, collectively, not to exceed 33.36 % of any such liability for a maximum aggregate liability of $2,000,000, and as limited hereby may be enforced at the option of USB against each Guarantor severally;

(B) The obligation of Ryll as to any liability covered hereby shall be several and not joint and several with the liability of Ryll not to exceed 33.36 % of any such liability for a maximum aggregate liability of $2,000,000, and as limited hereby may be enforced at the option of USB against each Guarantor severally;

(C) The obligation of Osman as to any liability covered hereby shall be several and not joint and several with the liability of Osman not to exceed 16.68 % of any such liability for a maximum aggregate liability of $1,000,000, and as limited hereby may be enforced at the option of USB against each Guarantor severally;

(D) The obligation of Stogel as to any liability covered hereby shall be several and not joint and several with the liability of Stogel not to exceed 8.34 % of any such liability for a maximum aggregate liability of $500,000, and as limited hereby may be enforced at the option of USB against each Guarantor severally; (E) The obligation of Furgerson and the Furgerson Trust, collectively, as to any liability covered hereby shall be several and not joint and several with the liability of Furgerson and the Furgerson Trust, collectively. not to exceed 8.34 % of any such liability for a maximum aggregate liability of $500,000, and as limited hereby may be enforced at the option of USB against each Guarantor severally;

(F) Biovest shall be jointly and severally liable for all of the Guaranteed Obligations hereunder. 2. A. Each Individual Guarantor severally (as more particularly defined and limited in maximum and percentage liability in Section 1 hereof) and Biovest jointly and severally absolutely, unconditionally, and irrevocably guarantees the full and prompt payment of any and all amounts which may become payable by Borrower under the Indemnity (the " Guaranteed Obligations" ); provided , however , that for purpose of determining the Guaranteed Obligations guaranteed by the Individual Guarantors, and for such purpose only, the Minimum Return Shortfall shall be determined: (i) as if the internal rate of return anticipated by the Investor, as reflected in the Financial Forecast, was an internal rate of return calculated without inclusion of any deductions of the Fund attributable to interest accruals in excess of payments, and (ii) without reference to any actual deductions taken by the Fund attributable to interest accruals in excess of payments. The terms " Minimum Return Shortfall" , " Investor" and


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" Financial Forecast" shall all have the meaning set forth in the Indemnity. Each Individual Guarantor severally (as more particularly defined and limited in maximum liability in Section 1 hereof) and absolutely, unconditionally, and irrevocably hereby also agrees to pay all reasonable costs, expenses, damages and attorneys' fees paid or incurred by USB, the CDE and Telesis in ende ...