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[COOLEY GODWARD LLP LETTERHEAD]

November 3, 1998


Cell Pathways Holdings, Inc.


                                                                     EXHIBIT 5.1

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Cell Pathways Holdings, Inc. (the "Registrant") of a
Registration Statement on Form S-8 (the "Registration Statement") with the
Securities and Exchange Commission covering the offering of up to 3,351,500
shares of the Registrant's Common Stock, $0.01 par value, pursuant to the
Registrant's Cell Pathways, Inc. 1997 Equity Incentive Plan, Cell Pathways, Inc.
1997 Non-Employee Directors' Stock Option Plan, Cell Pathways, Inc. Employee
Stock Purchase Plan, Tseng Labs, Inc. 1991 Stock Option Plan, Tseng Labs, Inc.
1995 Stock Option Plan and Tseng Labs, Inc. 1991 Special Directors Stock Option
Plan (collectively, the "Plans") (the "Shares").

In connection with this opinion, we have (i) examined the Registration Statement
and the related Prospectus and (ii) reviewed the Registrant's Certificate of
Incorporation and Bylaws, as amended, and such other documents, records,
certificates, memoranda and other instruments as we deem necessary as a basis
for this opinion. We also have assumed the genuineness and authenticity of all
documents submitted to us as originals, the conformity to originals of all
documents submitted to us as copies thereof and the due execution and delivery
of all documents where due execution and delivery are a prerequisite to the
effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when issued and sold in accordance with the Plans, the
Registration Statement and the related Prospectus, will be validly issued, fully
paid and nonassessable (except as to shares issued pursuant to certain deferred
payment arrangements, which will be fully paid and nonassessable when such
deferred payments are made in full).

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

Cooley Godward LLP



By:    /s/ James C. T. Linfield                             
       --------------------------
        James C. T. Linfield