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                                                                   EXHIBIT 5.1

                               PALMER & DODGE LLP
                                One Beacon Street
                           Boston, Massachusetts 02108

Telephone: (617) 573-0100                            Facsimile: (617) 227-4420


                                  July 19, 1996


GelTex Pharmaceuticals, Inc.
303 Bear Hill Road
Waltham, MA 02154

Ladies and Gentlemen:

         We are rendering this opinion in connection with the Registration
Statement on Form S-8 (the "Registration Statement") filed by GelTex
Pharmaceuticals, Inc. (the "Company") with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Act"), on or about the date hereof. The Registration Statement relates to
1,705,585 shares (the "Shares") of the Company's Common Stock, $0.01 par value,
offered pursuant to the provisions of the Company's 1992 Equity Incentive Plan
(the "Plan").

         We have acted as your counsel in connection with the preparation of the
Registration Statement and are familiar with the proceedings taken by the
Company in connection with the authorization of the issuance and sale of the
Shares. We have examined all such documents as we consider necessary to enable
us to render this opinion.

         Based upon the foregoing, we are of the opinion that when issued in
accordance with the terms of the Plan, the Shares will be duly authorized,
validly issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion as a part of the
Registration Statement and to the reference to our firm under Item 5 thereof.

                                                     Very truly yours,


                                                     /s/ Palmer & Dodge LLP
                                                     PALMER & DODGE LLP


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