POWER OF ATTORNEY

	KNOW ALL MEN BY THESE PRESENTS, that the person whose signature
appears below hereby constitutes and appoints Walter C. Herlihy,
 Daniel W. Muehl, Elizabeth R. Gunning, Lawrence S. Wittenberg
 and Robert E. Puopolo, and any one of them acting singly, the true
 and lawful attorneys-in-fact and agents, with full power of
substitution and resubstitution, for the undersigned and in the
 undersigneds name, place and stead, in any and all capacities
(until revoked in writing) to sign any and all instruments,
certificates and documents required to be executed on behalf of
 the undersigned as an individual or on behalf of the
undersigneds holding company, as the case may be, relating to
securities, or transactions in securities, of Repligen Corporation,
pursuant to sections 13 and 16 of the Securities Exchange Act
of 1934, as amended (the Exchange Act), and any and all
regulations promulgated thereunder, and to file the same, with
all exhibits thereto, and any other documents in connection
therewith, with the Securities and Exchange Commission, and with
any other entity when and if such is mandated by the Exchange Act
or by the By-laws of the National Association of Securities
Dealers, granting unto said attorneys-in-fact and agents full
power and authority to do and perform each and every act and
thing requisite and necessary fully to all intents and purposes
as the undersigned might or could do in person thereby ratifying
 and confirming all that said attorneys-in-fact and agents, or
their substitute or substitutes, may lawfully do or cause to be
 done by virtue hereof.

	IN WITNESS WHEREOF, this Power of Attorney has been
 signed as of 5/31/2006.




						Signature:	/s/ James Rusche
						Name:		James Rusche
         (please print)