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                                                                     EXHIBIT 5.1
 
                          [COOLEY GODWARD LETTERHEAD]
 
February 11, 1999
Cerus Corporation
2525 Stanwell Drive, Suite 300
Concord, CA 94520
Ladies and Gentlemen:
 
        You have requested our opinion with respect to certain matters in
connection with the filing by Cerus Corporation, a Delaware corporation (the
"Company"), of a Registration Statement on Form S-3 (the "Registration
Statement") with the Securities and Exchange Commission (the "Commission") on
February 11, 1999 covering the offering of a total of up to 2,300,000 shares of
Cerus' common stock with a par value of $0.01 (including 300,000 shares of
common stock for which the underwriters will be granted an over-allotment
option)(the "Shares").
 
        In connection with this opinion, we have examined and relied upon the
Registration Statement and related Prospectus included therein, Cerus' 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. We have assumed the genuineness and
authenticity of all documents submitted to us as originals, and the conformity
to originals 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 sold in accordance with the Registration
Statement, 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
 
                                          /s/ Andrea Vachss
                                          --------------------------------------
                                          Andrea Vachss