Agreement

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                                                               Exhibit 10.15

Siegfried

                                                  SIEGFRIED (USA), INC.

                                                  33 Industrial Park Road

                                                  Pennsville, NJ 08070

                                                  USA

                                                  +1 616-895-4553

                                                  +1 616-835-0321mobile

                                                  +1 419-710-5687 fax

                                                  scott.powers@siegfried-usa.com

          Confidential Materials omitted and filed separately with the

         Securities and Exchange Commission. Asterisks denote omissions.

January 26, 2005

Dr. John E. Bishop

Vice President, Pharmaceutical Sciences & Manufacturing

Momenta Pharmaceuticals, Inc.

675 West Kendall Street

Cambridge, MA 02142

Dear John:

Reference is made to the Development and Production Agreement for Active

Pharmaceutical Ingredient between Siegfried (USA), Inc. and Siegfried Ltd.

("SIEGFRIED") and Momenta Pharmaceuticals, Inc. ("MOMENTA") dated October 10,

2003, as amended by Letter Agreements dated February 14, 2004, May 17, 2004 and

September 29, 2004 (the "Agreement"). Capitalized terms used herein and not

otherwise defined shall have the meanings given such terms in the Agreement.

MOMENTA hereby ratifies SIEGFRIED's engagement of the two subcontractors

identified below in connection with the testing ("[**] TESTING") of [**] used by

SIEGFRIED in the manufacture of those quantities of the PRODUCT to be used by

MOMENTA in support of the ANDA covering the PRODUCT. The [**] TESTING shall be

deemed to have been conducted under the terms and conditions of the Agreement,

with the following modifications:

     1.   SIEGFRIED has contracted the services of [**] to perform (a) the [**]

          testing of [**], as per the previously provided quotation and scope of

          work received from [**], and (b) the [**] testing of [**].

          SIEGFRIED had previously approved [**] as a contract laboratory per

          SIEGFRIED's internal procedures.

     2.   SIEGFRIED has contracted the services of [**] to perform [**] testing

          [**] out of the facilities of [**]. SIEGFRIED had previously approved

          this facility of [**] as a contract laboratory per SIEGFRIED's

          internal procedures.

     3.   MOMENTA shall pay SIEGFRIED the actual costs of [**] TESTING as