Prospectus Supplement Filed pursuant to Rule 424(b)(3)
Registration No. 333-138257

PROSPECTUS SUPPLEMENT NO. 23

DATED JULY 14, 2008

(To Prospectus Dated November 17, 2006)

BIOVEST INTERNATIONAL, INC.

18,000,000 Shares of Common Stock

This prospectus supplement supplements information contained in, and should be read in conjunction with, that certain Prospectus, dated November 17, 2006, of Biovest International, Inc. (the “Company”) as supplemented by Supplement No. 22 thereto dated June 6, 2008, Supplement No. 21 thereto dated May 16, 2008, Supplement No. 20 thereto dated April 17, 2008, Supplement No. 19 thereto dated February 15, 2008, Supplement No. 18 thereto dated February 7, 2008, Supplement No. 17 thereto dated January 28, 2008, Supplement No. 16 thereto dated January 22, 2008, Supplement No. 15 thereto dated December 31, 2007, Supplement No. 14 thereto dated December 31, 2007, Supplement No. 13 thereto dated November 2, 2007, Supplement No. 12 thereto dated October 18, 2007, Supplement No. 11 thereto dated September 11, 2007, Supplement No. 10 thereto dated August 15, 2007, Supplement No. 9 thereto dated June 29, 2007, Supplement No. 8 thereto dated June 14, 2007, Supplement No. 7 thereto dated May 15, 2007, Supplement No. 6 thereto dated April 19, 2007, Supplement No. 5 thereto dated March 28, 2007, Supplement No. 4 thereto dated February 14, 2007, Supplement No. 3 thereto dated January 19, 2007, Supplement No. 2 thereto dated December 29, 2006 and Supplement No. 1 thereto dated December 14, 2006. This prospectus supplement is not complete without, and may not be delivered or used except in connection with, the original Prospectus and Supplement Nos. 1 through 22 thereto. The Prospectus relates to the public sale, from time to time, of up to 18,000,000 shares of our common stock by the selling shareholders identified in the Prospectus.

The information attached to this prospectus supplement modifies and supersedes, in part, the information in the Prospectus, as supplemented. Any information that is modified or superseded in the Prospectus shall not be deemed to constitute a part of the Prospectus, except as modified or superseded by this prospectus supplement or Prospectus Supplement Nos. 1 through 22.

This prospectus supplement includes the attached Form 8-K filed on July 14, 2008, as filed by us with the Securities and Exchange Commission.

We may amend or supplement the Prospectus from time to time by filing amendments or supplements as required. You should read the entire Prospectus and any amendments or supplements carefully before you make an investment decision.

The Securities and Exchange Commission and state securities regulators have not approved or disapproved these securities or determined if this Prospectus Supplement (or the original Prospectus dated November 17, 2006, as previously supplemented) is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is July 14, 2008.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): July 14, 2008

BIOVEST INTERNATIONAL, INC.

(Exact name of Registrant as Specified in its Charter)

377 Plantation Street

Worcester, Massachusetts 01605

(Address of Principal Executive Offices; Zip Code)

Registrant’s telephone number, including area code: (508) 793-0001

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

BIOVEST INTERNATIONAL, INC.

FORM 8-K

Effective May 1, 2008, the practice of Aidman, Piser & Company, P.A. (“Aidman Piser”), Biovest International, Inc.’s (the “Company”) independent registered public accounting firm, was acquired by Cherry, Bekaert & Holland, L.L.P. (“Cherry Bekaert”) in a transaction pursuant to which Aidman Piser merged its operations into Cherry Bekaert and certain of the professional staff and partners of Aidman Piser joined Cherry Bekaert either as employees or partners of Cherry Bekaert and will continue to practice as members of Cherry Bekaert. On July 14, 2008, and concurrently with the resignation of Aidman Piser, the Company, through and with the approval of the Audit Committee of the Company’s Board of Directors, engaged Cherry Bekaert as its independent registered public accounting firm.

Prior to engaging Cherry Bekaert, the Company did not consult with Cherry Bekaert regarding the application of accounting principles to a specific completed or contemplated transaction or regarding the type of audit opinions that might be rendered by Cherry Bekaert on the Company’s financial statements, and Cherry Bekaert did not provide any written or oral advice that was an important factor considered by the Company in reaching a decision as to any such accounting, auditing or financial reporting issue.

The reports of Aidman Piser regarding the Company’s financial statements for the fiscal years ended September 30, 2007 and 2006 did not contain any adverse opinion or disclaimer of opinion and were not qualified or modified as to uncertainty, audit scope or accounting principles, except that, with respect to the Company’s financial statements for the fiscal years ended September 30, 2007 and 2006, substantial doubt was raised as to the Company’s ability to continue as a going concern. During the years ended September 30, 2007 and 2006, and during the period from the end of the most recently completed fiscal year through July 14, 2008, the date of resignation, there were no disagreements with Aidman Piser on any matter of accounting principles or practices, financial statement disclosure or auditing scope or procedures, which disagreements, if not resolved to the satisfaction of Aidman Piser would have caused it to make reference to such disagreement in its reports.

The Company provided Aidman Piser with a copy of this Current Report on Form 8-K prior to its filing with the Securities and Exchange Commission and requested that Aidman Piser furnish the Company with a letter addressed to the Securities and Exchange Commission stating whether it agrees with above statements and, if it does not agree, the respects in which it does not agree. A copy of the letter, dated July 14, 2008, is filed as Exhibit 16.1 (which is incorporated by reference herein) to this Current Report on Form 8-K.

See the Exhibit Index set forth below for a list of exhibits included with this Form 8-K.

2

Signature

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunder duly authorized.

Date: July 14, 2008

3

EXHIBIT INDEX

4

Exhibit 16.1

July 14, 2008

Securities and Exchange Commission

100 F Street, NE

Washington, DC 20549

Dear Ladies and Gentlemen:

We are the former independent registered public accounting firm for Biovest International, Inc. (the “Company”). We have read the Company’s disclosure set forth in Item 4.01 “Changes in Registrant’s Certifying Accountant” of the Company’s Current Report on Form 8-K dated July 14, 2008 (the “Current Report”) and are in agreement with the disclosure in the Current Report, insofar as it pertains to our firm.