1



                                                                       EXHIBIT 5

                               PALMER & DODGE LLP
                                One Beacon Street
                           Boston, Massachusetts 02108

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



                                    August 3, 1998

GelTex Pharmaceuticals, Inc.
Nine Fourth Avenue
Waltham, MA 02451

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
35,000 shares (the "Shares") of the Company's Common Stock, $0.01 par value,
offered pursuant to the provisions of the Company's 1995 Director Stock Option
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.

                                    Very truly yours,


                                    /s/ Palmer & Dodge LLP
                                    PALMER & DODGE LLP