Exhibit 16.1

June 14, 2002

Office of the Chief Accountant
Securities and Exchange Commission
450 Fifth Street, N.W.
Washington, D.C. 20549
Fax Number: (202) 942-9656

Dear Sir/Madam:

We have read the second and third paragraphs of Item 4 included in the Form 8-K
dated June 19, 2002 of Repligen Corporation to be filed with the Securities and
Exchange Commission and are in agreement with the statements contained therein.

Very truly yours,


/s/ Arthur Andersen LLP

Arthur Andersen LLP