<Page>

                                                               Exhibit 10.34.2

                                 THIRD AMENDMENT
                        CONTRACT MANUFACTURING AGREEMENT

THIS AMENDMENT NUMBER 3 dated as of December 8, 2003 (the "3rd Amendment") is
made by and between The Dow Chemical Company, a Delaware corporation ("Dow") and
Genzyme Corporation, a Massachusetts corporation ("Genzyme"). The parties wish
to amend the Contract Manufacturing Agreement they entered into effective
September 4, 2001 ("Agreement") as hereafter set forth. Capitalized terms not
defined herein shall have the respective meanings ascribed to them in the
Agreement.

                                    RECITALS

WHEREAS, Dow and Genzyme have previously entered into the Agreement and
subsequent Amendment One to this agreement, whereby Dow, in connection with its
obligation to manufacture and supply Product to Genzyme, presently maintains an
annual Product manufacturing capacity level at the Dow Facility of [**] and
Genzyme has committed to purchase a minimum of [**];

WHEREAS, Genzyme under Amendment Two to this agreement increased its purchase
commitment to a minimum of [**] and Dow was willing to supply this additional
quantity.

WHEREAS, Genzyme and Dow hereby acknowledge that pursuant to Section 3.1, at a
minimum, the Agreement shall extend for an additional [**],

NOW THEREFORE, in consideration of the premises and of the covenants herein
contained, Dow and Genzyme hereby agree as follows:

  1. Genzyme commits to purchase a minimum of [**] and Dow agrees to supply this
     volume.

  2. The purchase price will be now calculated as follows:

     [**]

  3. Genzyme may request additional volumes in this period should they so
     desire. Dow plans to hold minimum finished product inventory during 2004.
     Thus such additional volumes and delivery dates are to be committed by
     Genzyme before [**].

     -    For all such additional quantities ordered for delivery in 2004, the
          price will be:

          [**]

     -    In the event that Genzyme requests further additional quantities
          during 2004 after [**], a three-month lead time will be required to
          restart the plant. Plant ramp up


[**] = Portions of this exhibit have been omitted pursuant to a confidential
treatment request. An unredacted version of this exhibit has been filed
separately with the commission.

<Page>

          rate and capacity will influence the rate that additional material can
          be made and delivered.

     -    In the event Genzyme requires the further additional product in a time
          frame sooner than 3 months then Dow shall make reasonable commercial
          efforts to supply these additional volumes sooner than the three
          months. Genzyme agrees to fund extraordinary start up costs that may
          be incurred by Dow to meet an accelerated schedule, subject to these
          costs being agreed by Genzyme before they are expended.

  4. It is the parties intent for Dow to deliver the first [**], Dow would be
     prepared to store any purchase Product for Genzyme (up to the maximum store
     capacity) should Genzyme want to delay shipment, without storage charges up
     to [**], Dow shall invoice Genzyme for any stored Product when the
     Product is ready for shipment and placed in storage pursuant to the mutual
     agreement of the parties, and title and risk of loss for Product in storage
     shall pass to Genzyme on date of the invoice.

This 3rd Amendment is intended to supplement the Agreement and except as
explicitly modified by the terms of this 3rd Amendment, the Agreement as
previously modified, shall continue in full force and effect. The Agreement as
previously modified, and this 3rd Amendment shall constitute the entire
agreement between the parties on the subject matter hereof and shall not be
amended except as provided in Article 33 of the Agreement.


     IN WITNESS WHEREOF, the parties hereto have caused this 3rd Amendment to be
executed by their duly authorized representatives effective as of the date set
forth above.

     THE DOW CHEMICAL COMPANY

     By:    /s/ George J. Biltz

     Name:  George J. Biltz

     Title: VP C & FC


     GENZYME CORPORATION

     By:    /s/ Henri A. Termeer

     Name:  Henri A. Termeer

     Title: CEO


[**] = Portions of this exhibit have been omitted pursuant to a confidential
treatment request. An unredacted version of this exhibit has been filed
separately with the commission.