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                                                                    Exhibit 5.1

                                June 19, 1997






IDEC Pharmaceuticals Corporation
11011 Torreyana Road
San Diego, California  92121


     Re:    IDEC Pharmaceuticals Corporation (the "Company")
            Registration Statement of an Aggregate of 950,000 Shares 
            of Common Stock


Ladies and Gentlemen:

        We refer to your Registration Statement on Form S-8 (the "Registration
Statement") under the Securities Act of 1933, as amended, of (i) Eight Hundred
Thousand (800,000) shares of common stock ("Common Stock") of IDEC
Pharmaceuticals Corporation (the "Company") issuable under the Company's Amended
and Restated 1988 Stock Option Plan, as amended and restated, (the "Option
Plan") and (ii) One Hundred Fifty Thousand (150,000) shares of Common Stock
issuable under the Company's 1995 Employee Stock Purchase Plan (the "Purchase
Plan"). We advise you that, in our opinion, when such shares have been issued
and sold pursuant to the applicable provisions of the Option Plan and/or the
Purchase Plan and in accordance with the Registration Statement, such shares
will be validly issued, fully paid and non-assessable shares of the Company's
Common Stock.

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


                                           Very truly yours,


                                           /s/ Brobeck, Phleger & Harrison LLP

                                           Brobeck, Phleger & Harrison LLP