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                                                                     EXHIBIT 5.1

                                August 31 , 1998

IDEC Pharmaceuticals Corporation
11011 Torreyana Road
San Diego, CA  92121

     Re:  IDEC Pharmaceuticals Corporation Registration Statement on Form S-8
          for 975,000 Shares of Common Stock and Related Stock Options

Ladies and Gentlemen:

     We have acted as counsel to IDEC Pharmaceuticals Corporation, a Delaware
corporation (the "Company"), in connection with the registration on Form S-8
(the "Registration Statement") under the Securities Act of 1933, as amended, of
(i) an additional 855,000 shares of common stock for issuance under the
Company's Amended and Restated 1988 Stock Option Plan (the "Incentive Plan") and
(ii) an additional 120,000 shares of common stock for issuance under the 1993
Non-Employee Directors Stock Option Plan (the "Non-Employee Directors Plan").

     This opinion is being furnished in accordance with the requirements of Item
8 of Form S-8 and Item 601(b)(5)(i) of Regulation S-K.

     We have reviewed the Company's charter documents and the corporate
proceedings taken by the Company in connection with the establishment and
amendment of the Incentive Plan and the Non-Employee Director Plan. Based on
such review, we are of the opinion that, if, as and when the shares of common
stock are issued and sold (and the consideration therefor received) pursuant to
(a) the provisions of option agreements duly authorized under the Incentive Plan
and in accordance with the Registration Statement, (b) duly authorized direct
stock issuances effected in accordance with the provisions of the Incentive Plan
and the Registration Statement or (c) the provisions of the Non-Employee
Director Plan and the option agreements authorized thereunder and in accordance
with the Registration Statement, such shares will be duly authorized, legally
issued, fully paid and non-assessable.

     We consent to the filing of this opinion letter as Exhibit 5.1 to the
Registration Statement.

     This opinion letter is rendered as of the date first written above and we
disclaim any obligation to advise you of facts, circumstances, events or
developments which hereafter may be brought to our attention and which may
alter, affect or modify the opinion expressed herein. Our opinion is expressly
limited to the matters set forth above and we render no opinion, whether by
implication or otherwise, as to any other matters relating to the Company, the
Incentive Plan or the Non-Employee Director Plan or the shares of common stock
issuable under such plans.

                                            Very truly yours,

                                            /s/ BROBECK, PHLEGER & HARRISON LLP

                                            BROBECK, PHLEGER & HARRISON LLP