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 September 30, 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
                                        November 12, 2002