<PAGE>   1

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549


                                 ---------------


                                    FORM 8-K

                                 CURRENT REPORT


                     PURSUANT TO SECTION 13 OR 15(D) OF THE

                         SECURITIES EXCHANGE ACT OF 1934


Date of Report (Date of earliest event reported)     SEPTEMBER 17, 1998
                                                --------------------------------

                        IDEC PHARMACEUTICALS CORPORATION
- --------------------------------------------------------------------------------
               (Exact name of registrant as specified in charter)


<TABLE>
<S>                                   <C>                       <C>
         DELAWARE                       0-19311                     33-0112644
- -----------------------------------------------------------------------------------
(State or other jurisdiction          (Commission                  (IRS Employer
     of incorporation)                File Number)              Identification No.)


11011 TORREYANA ROAD, SAN DIEGO, CALIFORNIA                           92121
- -----------------------------------------------------------------------------------
(Address of principal executive offices)                            (Zip Code)
</TABLE>



Registrant's telephone number, including area code   (619) 550-8500
                                                  ------------------------------

                                      NONE
- --------------------------------------------------------------------------------
         (Former name or former address, if changed since last report.)


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Item 5. Other Events.

     On September 17, 1998, the Board of Directors of IDEC Pharmaceuticals
Corporation (the "Company") amended its Rights Agreement, dated as of July 22,
1997, between the Company and Chase Mellon Shareholder Services LLC (the "Rights
Agent") (the "Agreement"), to delete "continuing director" provisions throughout
the Agreement.

     The Form of First Amendment to the Rights Agreement between the Company and
the Rights Agent, eliminating "continuing director" provisions, is attached
hereto as an exhibit and is incorporated herein by reference. The foregoing
description of the "continuing director" provisions is qualified in its entirety
by reference to such exhibit.



Item 7. Exhibits.


<TABLE>
<S>                 <C>

        4.1         Form of First Amendment to the Rights Agreement between the 
                    Company and Chase Mellon Shareholder Services LLC.
</TABLE>



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                                    SIGNATURE

     Pursuant to the requirements of the Securities Exchange Act of 1934, the
Registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.



                                              IDEC PHARMACEUTICALS CORPORATION



DATE:  December 31, 1998                      By: /s/ Phillip M Schneider
                                                 -------------------------------
                                                  Name:  Phillip M. Schneider
                                                  Title: Vice President and 
                                                         Chief Financial Officer


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                                  EXHIBIT INDEX


<TABLE>
<CAPTION>
EXHIBIT
NUMBER                                DOCUMENT DESCRIPTION
- -------                               --------------------
<S>            <C>
   4.1         Form of First Amendment to the Rights Agreement between the Company 
               and Chase Mellon Shareholder Services LLC.
</TABLE>







<PAGE>   1

                                                                     EXHIBIT 4.1


                     FIRST AMENDMENT TO THE RIGHTS AGREEMENT
             AND CERTIFICATION OF COMPLIANCE WITH SECTION 27 THEREOF


     Pursuant to Section 27 of the Rights Agreement (the "Agreement") dated as 
of July 22, 1997, between IDEC PHARMACEUTICALS CORPORATION, a Delaware
corporation (the "Company"), and CHASE MELLON SHAREHOLDER SERVICES, LLC, a
national banking association (the "Rights Agent"), the Company and the Rights
Agent hereby amend the Agreement as of September 17, 1998, as provided below.

     1.   Certain Definitions. Section 1 of the Agreement shall be amended as
follows:

          (a)  The definition of Acquiring Person shall be amended as follows:
(1) the phrase "(upon approval by a majority of the Continuing Directors (as
such term is hereinafter defined))" shall be deleted in subsection (a)(ii) and
(2) the phrase "a majority of the Continuing Directors" shall be replaced with
the phrase "the Board of Directors of the Company" in subsection (a)(iii).

          (b)  The definition of Beneficial Owner and to "beneficially own"
shall be amended by deleting the phrase ", upon the affirmative vote of a
majority of Continuing Directors," from subsection (c)(ii)(A). 

          (c)  The definition of Continuing Directors shall be deleted. 

     2.   Issue of Rights Certificates. Section 3(a) of the Agreement
 shall be
amended by deleting the phrase "(upon approval by a majority of the Continuing
Directors)" in the first sentence of the paragraph.

     3.   Adjustment of Purchase Price, Number of Shares or Number of Rights.
Section 11 shall be amended as follows: 

          (a)  The phrase ", upon approval by a majority of the Continuing
Directors," which appears once in the second paragraph of subsection (a)(ii),
three times in subsection (a)(iv), once in subsection (b), once in subsection
(c), once in subsection (d)(i) and once in subsection (d)(ii) shall be deleted.

          (b)  The phrase "at least a majority of the Continuing Directors" in
subsection (a)(iii) shall be replaced with the phrase "the Board of Directors of
the Company." 

          (c)  The phrase "and approved by a majority of the Continuing
Directors" in subsection (a)(iv) shall be deleted. 

          (d)  The phrase ", subject to approval by a majority of the Continuing
Directors," in subsection (a)(iv) shall be deleted. 


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     4.   Consolidation, Merger or Sale or Transfer of Assets or Earning Power.
Section 13(d) shall be amended by deleting the phrase, "upon approval by a
majority of the Continuing Directors," in the second sentence of the paragraph.

     5.   Fractional Rights and Fractional Shares. Section 14(a) shall be
amended by deleting the phrase ", upon approval by a majority of the Continuing
Directors," in the third to last and second to last sentences of the paragraph.


     6.   Issuance of New Rights Certificates. Section 22 shall be amended by
deleting the phrase ", upon approval by a majority of the Continuing Directors,"
in the first and second sentences of the paragraph. 

     7.   Redemption. Section 23 shall be amended as follows: 

          (a)  The phrase "by a majority of the Continuing Directors" that
appears three times in the first sentence of subsection (a) shall be replaced 
with the phrase "the Board of Directors of the Company".

          (b)  The phrase ", upon the approval by the majority of the Continuing
Directors" in the last sentence of subsection (a) shall be deleted.

     8.   Exchange. Section 24 shall be amended as follows:

          (a)  The phrase "a majority of Continuing Directors" in the first
sentence of subsection (a) shall be replaced with the phrase "the Board of
Directors of the Company".

          (b)  The phrase "and upon approval by a majority of the Continuing
Directors" and the phrase ", and approved by a majority of the Continuing
Directors," in the first sentence of subsection (d) shall both be deleted. 

          (c)  The phrase ", upon approval by a majority of the Continuing
Directors," in the second to last sentence of subsection (d) shall be deleted.

     9.   Supplements and Amendments. Section 27 shall be amended as follows:

          (a)  The phrase ", upon approval by a majority of the Continuing
Directors," in the first sentence of the paragraph shall be deleted.

          (b)  The phrase "and approval by a majority of the Continuing
Directors," in the second and last sentences of the paragraph shall be deleted.

     10.  Determination and Actions by the Board of Directors. Section 29 shall
be amended as follows:

          (a)  The phrase "(and, where specifically provided for herein, only
upon approval by a majority of the Continuing Directors)" in the second sentence
of the paragraph shall be deleted.


                                       2


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          (b)  The phrase "(or, where specifically provided for herein, upon
approval by a majority of the Continuing Directors)" in the last sentence of the
paragraph shall be deleted. 

          (c)  The phrase "or the Continuing Directors" in the last sentence of
subsection (a) shall be deleted. 

     11.  Severability. Section 31 shall be amended by deleting the phrase ",
upon approval by a majority of the Continuing Directors," in the paragraph.

     The undersigned officer of the Company, being an appropriate officer of the
Company and authorized to do so by resolution of the board of directors of the
Company dated as of September 17, 1998, hereby certifies to the Rights Agent
that these amendments are in compliance with the terms of Section 27 of the
Agreement.

                                             IDEC PHARMACEUTICALS CORPORATION



                                             By:
                                                --------------------------------
                                             Name:  Phillip M. Schneider
                                             Title: Vice President and 
                                                    Chief Financial Officer


Acknowledged and Agreed:

CHASE MELLON SHAREHOLDER SERVICES, LLC
As Rights Agent



By:
   --------------------------------
   Name:
   Title:


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