EXHIBIT 10.45

                                     DRAFT 3
                              Repligen Corporation
                               One Kendall Square
                                  Building 700
                         Cambridge, Massachusetts 02139
                                 (617) 225-6000

Mr. Daniel Parker Witt
288 Essex Street
Hamilton, MA  01982

Dear Mr. Witt:

        This is to confirm that you will be appointed Vice President, Business
Development of Repligen Corporation effective 14 March, 1996.

        Our understanding of the terms of your employment are as follows:

        1. You will receive a base annual salary of $115,000.  Bonuses, based
           upon performance, may be given in the discretion of the Board of 
           Directors.

        2. You will be awarded options for 60,000 shares of common stock of
           Repligen Corporation which will vest evenly over a five-year
           period and which will be governed by Repligen's stock option
           plans. Notwithstanding the foregoing, 50% of any unvested options
           shall vest immediately upon notice of termination of your
           employment or a change in control of the Corporation.

        3. You are entitled to all health, medical and life insurance benefits 
           that are provided, from time to time, by Repligen Corporation.

        4. Your employment at Repligen may terminate, with or without cause,
           upon 30 days' prior written notice from the Board of Directors or
           from you. If your employment is terminated, you will be entitled
           to severance remuneration of base salary at the rate then in
           effect, payable monthly in arrears, until you find employment,
           but for no longer than 6 months.

        5. You agree to execute standard noncompetition and nondisclosure 
           agreements as soon as possible.

        We look forward to an exciting collaboration between you, the Board of
Directors, and all the employees at Repligen.

        Could you kindly indicate your agreement to the above by signing in the
space provided below and returning one copy to me.

                                     REPLIGEN CORPORATION:

                                     By:

                                     Name:

                                     Title:

                                     Date:


I AGREE TO THE ABOVE:


- --------------------------------------

DATED: