EXHIBIT 5.1 [COOLEY GODWARD LLP LETTERHEAD] October 2, 2000 Rigel Pharmaceuticals, Inc. 240 East Grand Avenue South San Francisco, CA 94080 Ladies and Gentlemen: You have requested our opinion with respect to certain matters in connection with the filing by Rigel Pharmaceuticals, Inc. (the "Company") of a Registration Statement on Form S-1 (the "Registration Statement") with the Securities and Exchange Commission (the "Commission") covering the underwritten public offering of up to 10,350,000 shares of common stock, including 9,000,000 shares to be sold by the Company (the "Company Shares") and 1,350,000 shares for which the Underwriters have been granted an over-allotment option (together with the Company Shares, the "Common Stock"). In connection with this opinion, we have (i) examined and relied upon the Registration Statement and related Prospectus, the Company's Amended and Restated Certificate of Incorporation and Bylaws and the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below and (ii) assumed that the shares of the Common Stock will be sold by the Underwriters at a price established by the Pricing Committee of the Company's Board of Directors. On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Common Stock, when sold and issued in accordance with the Registration Statement and related Prospectus, will be validly issued, fully paid and nonassessable. We consent to the reference to our firm under the caption "Legal Matters" in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement. Very truly yours, Cooley Godward LLP *By: /s/ SUZANNE SAWOCHKA HOOPER -------------------------------------- Suzanne Sawochka Hooper, Esq.