Exhibit 99.1

                            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 of Repligen Corporation (the "Company")
on Form 10-Q for the period ending December 31, 2002 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), I, Walter
C. Herlihy, President, Chief Executive Officer and Principal Financial and
Accounting Officer of the Company, certify, pursuant to 18 U.S.C. ss.1350, as
adopted pursuant to ss.906 of the Sarbanes-Oxley Act, 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 result of operations of the Company.


                            /s/ Walter C. Herlihy
                            ------------------------------------------
                            Chief Executive Officer and President
                            Principal Financial and Accounting Officer
                            February 7, 2003


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