GUARANTY

Exhibit 10.9

GUARANTY

FOR VALUE RECEIVED, and in consideration of note purchases from, or credit otherwise extended or to be extended by LV Administrative Services, Inc., as agent (“ Agent ”), Valens U.S. SPV I, LLC (“ Valens U.S. ”) and Valens Offshore SPV II, Corp. (“ Valens Offshore ” and together with Agent and Valens U.S., the “ Creditor Parties ”), to or for the account of Biovest International, Inc., a Delaware corporation (the “ Company ”), from time to time and at any time and for other good and valuable consideration and to induce Creditor Parties, in their discretion, to purchase such notes or make other extensions of credit and to make or grant such renewals, extensions, releases of collateral or relinquishments of legal rights as Creditor Parties may deem advisable, each of the undersigned (and each of them if more than one, the liability under this Guaranty being joint and several) (jointly and severally referred to as “Guarantors “ or “the undersigned”) unconditionally guaranties to Agent and each other Creditor Party, and their respective successors, endorsees and assigns the prompt payment when due (whether by acceleration or otherwise) of all present and future obligations and liabilities of any and all kinds of the Company to Creditor Parties and of all instruments of any nature evidencing or relating to any such obligations and liabilities upon which the Company or one or more parties and the Company is or may become liable to Creditor Parties, whether incurred by the Company as maker, endorser, drawer, acceptor, guarantors, accommodation party or otherwise, and whether due or to become due, secured or unsecured, absolute or contingent, joint or several, and however or whenever acquired by Creditor Parties, whether arising under, out of, or in connection with (i) that certain Note Purchase Agreement dated as of the date hereof by and between Valens U.S. and the Company (as amended, restated, modified and/or supplemented from time to time, the “ Valens U.S. Purchase Agreement ”), (ii) that certain Note Purchase Agreement dated as of the date hereof by and between Valens Offshore and the Company (as amended, restated, modified and/or supplemented from time to time, the “ Valens Offshore Purchase Agreement ” and together with the Valens U.S. Purchase Agreement, each a “ Purchase Agreement ” and collectively, the “ Purchase Agreements ”), and (iii) each Related Agreement referred to in each Purchase Agreement (the Purchase Agreements and each Related Agreement, as each may be amended, modified, restated and/or supplemented from time to time, are collectively referred to herein as the “ Documents ”), or any documents, instruments or agreements relating to or executed in connection with the Documents or any documents, instruments or agreements referred to therein or otherwise, or any other obligations or liabilities of the Company to Creditor Parties, whether now existing or hereafter arising, direct or indirect, liquidated or unliquidated, absolute or contingent, due or not due and whether under, pursuant to or evidenced by a note, agreement, guaranty, instrument or otherwise (all of which are herein collectively referred to as the “ Obligations ”), and irrespective of the genuineness, validity, regularity or enforceability of such Obligations, or of any instrument evidencing any of the Obligations or of any collateral therefor or of the existence or extent of such collateral, and irrespective of the allowability, allowance or disallowance of any or all of the Obligations in any case commenced by or against the Company under Title 11, United States Code, including, without limitation, obligations or indebtedness of the Company for post-petition interest, fees, costs and charges that would have accrued or been added to the Obligations but for the commencement of such case. Terms not otherwise defined herein shall have the meaning assigned such terms in the Purchase Agreements. In furtherance of the foregoing, the undersigned hereby agrees as follows:

1. No Impairment . Creditor Parties may at any time and from time to time, either before or after the maturity thereof, without notice to or further consent of the undersigned, extend the time of payment of, exchange or surrender any collateral for, renew or extend any of the Obligations or increase or decrease the interest rate thereon, or any other agreement with the Company or with any other party to or person liable on any of the Obligations, or interested therein, for the extension, renewal, payment, compromise, discharge or release thereof, in whole or in part, or for any modification of the terms thereof or of any agreement between any Creditor Party and the Company or any such other party or person, or make any election of rights any Creditor Party may deem desirable under the United States Bankruptcy Code, as amended, or any other federal or state bankruptcy, reorganization, moratorium or insolvency law relating to or affecting the enforcement of creditors’ rights generally (any of the foregoing, an “ Insolvency Law ”) without in any way impairing or affecting this Guaranty. This Guaranty shall be effective regardless of the subsequent incorporation, merger or consolidation of the Company, or any change in the composition, nature, personnel or location of the Company and shall extend to any successor entity to the Company, including a debtor in possession or the like under any Insolvency Law.

2. Guaranty Absolute . Each of the undersigned jointly and severally guarantees that the Obligations will be paid strictly in accordance with the terms of the Documents and/or any other document, instrument or agreement creating or evidencing the Obligations, regardless of any law, regulation or order now or hereafter in effect in any jurisdiction affecting any of such terms or the rights of the Company with respect thereto. Guarantors hereby knowingly accept the full range of risk encompassed within a contract of “continuing guaranty” which risk includes the possibility that the Company will contract additional obligations and liabilities for which Guarantors may be liable hereunder after the Company’s financial condition or ability to pay its lawful debts when they fall due has deteriorated, whether or not the Company has properly authorized incurring such additional obligations and liabilities. The undersigned acknowledge that (i) no oral representations, including any representations to extend credit or provide other financial accommodations to the Company, have been made by any Creditor Party to induce the undersigned to enter into this Guaranty and (ii) any extension of credit to the Company shall be governed solely by the provisions of the Documents. The liability of each of the undersigned under this Guaranty shall be absolute and unconditional, in accordance with its terms, and shall remain in full force and effect without regard to, and shall not be released, suspended, discharged, terminated or otherwise affected by, any circumstance or occurrence whatsoever, including, without limitation: (a) any waiver, indulgence, renewal, extension, amendment or modification of or addition, consent or supplement to or deletion from or any other action or inaction under or in respect of the Documents or any other instruments or agreements relating to the Obligations or any assignment or transfer of any thereof, (b) any lack of validity or enforceability of any Document or other documents, instruments or agreements relating to the Obligations or any assignment or transfer of any thereof, (c) any furnishing of any additional security to Agent, for the benefit of Creditor Parties, or its assignees or any acceptance thereof or any release of any security by Agent or its assignees, (d) any limitation on any party’s liability or obligation under the Documents or any other documents, instruments or agreements relating to the Obligations or any assignment or transfer of any thereof or any invalidity or unenforceability,

2

in whole or in part, of any such document, instrument or agreement or any term thereof, (e) any bankruptcy, insolvency, reorganization, composition, adjustment, dissolution, liquidation or other like proceeding relating to the Company, or any action taken with respect to this Guaranty by any trustee or receiver, or by any court, in any such proceeding, whether or not the undersigned shall have notice or knowledge of any of the foregoing, (f) any exchange, release or nonperfection of any collateral, or any release, or amendment or waiver of or consent to departure from any guaranty or security, for all or any of the Obligations or (g) any other circumstance which might otherwise constitute a defense available to, or a discharge of, the undersigned. Any amounts due from the undersigned to any Creditor Party shall bear interest until such amounts are paid in full at the highest rate then applicable to the Obligations. Obligations include post-petition interest whether or not allowed or allowable.

3. Waivers .

(a) This Guaranty is a guaranty of payment and not of collection. No Creditor Party shall be under any obligation to institute suit, exercise rights or remedies or take any other action against the Company or any other person or entity liable with respect to any of the Obligations or resort to any collateral security held by it to secure any of the Obligations as a condition precedent to the undersigned being obligated to perform as agreed herein and each of the Guarantors hereby waives any and all rights which it may have by statute or otherwise which would require any Creditor Party to do any of the foregoing. Each of the Guarantors further consents and agrees that Creditor Parties shall be under no obligation to marshal any assets in favor of Guarantors, or against or in payment of any or all of the Obligations. The undersigned hereby waives all suretyship defenses and any rights to interpose any defense, counterclaim or offset of any nature and description which the undersigned may have or which may exist between and among any Creditor Party, the Company and/or the undersigned with respect to the undersigned’s obligations under this Guaranty, or which the Company may assert on the underlying debt, including but not limited to failure of consideration, breach of warranty, fraud, payment (other than cash payment in full of the Obligations), statute of frauds, bankruptcy, infancy, statute of limitations, accord and satisfaction, and usury.

(b) Each of the undersigned further waives (i) notice of the acceptance of this Guaranty, of the extensions of credit, and of all notices and demands of any kind to which the undersigned may be entitled, including, without limitation, notice of adverse change in the Company’s financial condition or of any other fact which might materially increase the risk of the undersigned and (ii) presentment to or demand of payment from anyone whomsoever liable upon any of the Obligations, protest, notices of presentment, non-payment or protest and notice of any sale of collateral security or any default of any sort.

(c) Notwithstanding any payment or payments made by the undersigned hereunder, or any setoff or application of funds of the undersigned by any Creditor Party, the undersigned shall not be entitled to be subrogated to any of the rights of any Creditor Party against the Company or against any collateral or guarantee or right of offset held by any Creditor Party for the payment of the Obligations, nor shall the undersigned seek or be entitled to seek any contribution or reimbursement from the Company in respect of payments made by the undersigned hereunder, until all amounts owing to Creditor Parties

3

by the Company on account of the Obligations are indefeasibly paid in full and Creditor Parties’ obligation to extend credit pursuant to the Documents has been irrevocably terminated. If, notwithstanding the foregoing, any amount shall be paid to the undersigned on account of such subrogation rights at any time when all of the Obligations shall not have been paid in full and Creditor Parties’ obligation to extend credit pursuant to the Documents shall not have been terminated, such amount shall be held by the undersigned in trust for Creditor Parties, segregated from other funds of the undersigned, and shall forthwith upon, and in any event within two (2) business days of, receipt by the undersigned, be turned over to Agent, for the ratable benefit of Creditor Parties, in the exact form received by the undersigned (duly endorsed by the undersigned to Agent, if required), to be applied against the Obligations, whether matured or unmatured, in such order as Agent may determine, subject to the provisions of the Documents. Any and all present and future obligations and liabilities of the Company to any of the unders