Charles River Laboratories International (CRL) 10-Q 2003 Q3 Quarterly report

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,
AS ADOPTED PURSUANT TO
SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

        In connection with the quarterly report on Form 10-Q for the period ended September 27, 2003 of Charles River Laboratories International, Inc. (the "Company") as filed with the Securities and Exchange Commission on the date hereof (the "Report"), the undersigned, James C. Foster, the Chairman, Chief Executive Officer and President, and Thomas F. Ackerman, Senior Vice President and Chief Financial Officer, each hereby certifies, to the best of his knowledge and pursuant to 18 U.S.C. Section 1350, that:

        (1)   the Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

        (2)   the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: October 30, 2003	/s/  JAMES C. FOSTER
	James C. Foster Chairman, Chief Executive Officer & President Charles River Laboratories International, Inc.
Dated: October 30, 2003	/s/  THOMAS F. ACKERMAN
	Thomas F. Ackerman Senior Vice President & Chief Financial Officer Charles River Laboratories International, Inc.

QuickLinks

Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002