<PAGE>

                                  UNITED STATES
                        SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549



                                  SCHEDULE 13G

                   UNDER THE SECURITIES EXCHANGE ACT OF 1934
                                (AMENDMENT NO. 1)*


                       IDEC Pharmaceuticals Corporation
- --------------------------------------------------------------------------------
                                (Name of Issuer)

                                 Common Stock
- --------------------------------------------------------------------------------
                        (Title of Class of Securities)

                                 449370-10-5
            --------------------------------------------------------
                                (CUSIP Number)

                               December 31, 1998
- --------------------------------------------------------------------------------
            (Date of Event Which Requires Filing of this Statement)
                                          

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

   /X/       Rule 13d-1(b)
   / /       Rule 13d-1(c)
   / /       Rule 13d-1(d) 


*The remainder of this cover page shall be filled out for a reporting 
person's initial filing on this form with respect to the subject class of 
securities, and for any subsequent amendment containing information which 
would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be 
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange 
Act of 1934 ("Act") or otherwise subject to the liabilities of that section 
of the Act but shall be subject to all other provisions of the Act (however, 
see the Notes).


                                      Page 1
                                    of 7 Pages


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                                     SCHEDULE 13G

CUSIP NO.449370-10-5                                          Page 2 of 7 Pages


- -------------------------------------------------------------------------------
 (1) NAMES OF REPORTING PERSONS
     I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

     Salomon Smith Barney Holdings Inc.
     22-1660266
- -------------------------------------------------------------------------------
 (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                                       (a)  / /
                                                                       (b)  / /
- -------------------------------------------------------------------------------
 (3) SEC USE ONLY



- -------------------------------------------------------------------------------
 (4) CITIZENSHIP OR PLACE OF ORGANIZATION                              Delaware


                                                                       
- -------------------------------------------------------------------------------
       NUMBER OF           (5) SOLE VOTING POWER                              0

        SHARES                                                                
                           ----------------------------------------------------
     BENEFICIALLY          (6) SHARED VOTING POWER                   1,401,796*

       OWNED BY
                           ----------------------------------------------------
        EACH               (7) SOLE DISPOSITIVE POWER                         0

      REPORTING                                                               
                           ----------------------------------------------------
       PERSON              (8) SHARED DISPOSITIVE POWER              1,401,796*

        WITH:
- -------------------------------------------------------------------------------
(9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON     1,401,796*



- -------------------------------------------------------------------------------
(10) CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE 
     INSTRUCTIONS)  / /


- -------------------------------------------------------------------------------
(11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)                    7.0%*



- -------------------------------------------------------------------------------
(12) TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)                             HC


                                                                             
- -------------------------------------------------------------------------------
 * Assumes exercise of certain securities held.

<PAGE>

                                     SCHEDULE 13G

CUSIP NO.449370-10-5                                          Page 3 of 7 Pages


- -------------------------------------------------------------------------------
 (1) NAMES OF REPORTING PERSONS
     I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

     Citigroup Inc.
     52-1568099
- -------------------------------------------------------------------------------
 (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)

                                                                       (a)  / /
                                                                       (b)  / /
- -------------------------------------------------------------------------------
 (3) SEC USE ONLY



- -------------------------------------------------------------------------------
 (4) CITIZENSHIP OR PLACE OF ORGANIZATION                              Delaware


                                                                       
- -------------------------------------------------------------------------------
       NUMBER OF           (5) SOLE VOTING POWER                              0

        SHARES                                                                
                           ----------------------------------------------------
     BENEFICIALLY          (6) SHARED VOTING POWER                   1,406,896*

       OWNED BY
                           ----------------------------------------------------
        EACH               (7) SOLE DISPOSITIVE POWER                         0

      REPORTING                                                               
                           ----------------------------------------------------
       PERSON              (8) SHARED DISPOSITIVE POWER              1,406,896*

        WITH:
- -------------------------------------------------------------------------------
(9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON     1,406,896*



- -------------------------------------------------------------------------------
(10) CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE 
     INSTRUCTIONS)  / /


- -------------------------------------------------------------------------------
(11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)                    7.0%*



- -------------------------------------------------------------------------------
(12) TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)                             HC


                                                                             
- -------------------------------------------------------------------------------
 * Assumes exercise of certain securities held.

<PAGE>

Item 1(a).     Name of Issuer:

               IDEC Pharmaceuticals Corporation

Item 1(b).     Address of Issuer's Principal Executive Offices:

               11011 Torreyana Road
               San Diego, CA 92121

Item 2(a).     Name of Person Filing:

               Salomon Smith Barney Holdings Inc. ("SSB Holdings")
               Citigroup Inc. ("Citigroup")

Item 2(b).     Address or Principal Office or, if none, Residence:

               The address of the principal office of SSB Holdings is:

               388 Greenwich Street
               New York, NY 10013

               The address of the principal office of Citigroup is:

               153 East 53rd Street
               New York, NY 10043

Item 2(c).     Citizenship or Place of Organization:

               SSB Holdings and Citigroup are Delaware corporations.

Item 2(d).     Title of Class of Securities:

               Common Stock

Item 2(e).     Cusip Number:
               449370-10-5


                                       Page 4
                                     of 7 Pages

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Item 3.      If this Statement is Filed Pursuant to Sections 240.13d-1(b) or
             240.13d-2(b) or (c), Check Whether the Person Filing is a(n):

             (a)  [ ] Broker or dealer registered under section 15 of the 
                      Act (15 U.S.C. 78o);

             (b)  [ ] Bank as defined in section 3(a)(6) of the Act 
                      (15 U.S.C. 78c);

             (c)  [ ] Insurance company as defined in section 3(a)(19) of 
                      the Act (15 U.S.C. 78c);

             (d)  [ ] Investment company registered under Section 8 of the 
                      Investment Company Act of 1940 (15 U.S.C. 80a-8);

             (e)  [ ] Investment adviser in accordance with Section
                      240.13d-1(b)(1)(ii)(E);

             (f)  [ ] Employee benefit plan or endowment fund in accordance 
                      with Section 240.13d-1(b)(1)(ii)(F);

             (g)  [X] Parent holding company or control person in accordance
                      with Section  240.13d-1(b)(1)(ii)(G);

             (h)  [ ] Savings association as defined in Section 3(b) of the 
                      Federal Deposit Insurance Act (12 U.S.C. 1813);

             (i)  [ ] Church plan that is excluded from the definition of an 
                      investment company under Section 3(c)(14) of the 
                      Investment Company Act of 1940 (15 U.S.C. 80a-3)

             (j)  [ ] Group, in accordance with Section 240.13d-1(b)(1)(ii)(J).


Item 4.   Ownership. (as of December 31, 1998)

          (a)  Amount beneficially owned: See item 9 of cover pages

          (b)  Percent of Class: See item 11 of cover pages

          (c)  Number of shares as to which the person has:

                  (i)   sole power to vote or to direct the vote:

                  (ii)  shared power to vote or to direct the vote:

                  (iii) sole power to dispose or to direct the disposition of:

                  (iv)  shared power to dispose or to direct the disposition of:


         See Items 5-8 of cover pages


                                       Page 5
                                     of 7 Pages

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Item 5.   Ownership of Five Percent or Less of a Class.

          If this statement is being filed to report the fact that as of the 
          date hereof the reporting person has ceased to be the beneficial owner
          of more than five percent of the class of securities, check the
          following [  ].

Item 6.   Ownership of More Than Five Percent on Behalf of Another Person.

          Not Applicable.

Item 7.   Identification and Classification of the Subsidiary which Acquired 
          the Security being Reported on by the Parent Holding Company.

          Citigroup is the sole stockholder of SSB Holdings.

Item 8.   Identification and Classification of Members of the Group.

          Not Applicable.

Item 9.   Notice of Dissolution of Group. 

          Not Applicable.



                                       Page 6
                                     of 7 Pages


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Item 10.  Certification.

          By signing below I certify that, to the best of my knowledge and 
belief, the securities referred to above were acquired and are held in the 
ordinary course of business and were not acquired and are not held for the 
purpose of or with the effect of changing or influencing the control of the 
issuer of the securities and were not acquired and are not held in connection 
with or as a participant in any transaction having that purpose or effect.

SIGNATURE.

          After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this statement is true, 
complete and correct.

Dated: January 19, 1999


                      SALOMON SMITH BARNEY HOLDINGS INC.



                      By: /s/ Howard M. Darmstadter
                         --------------------------------------------
                         Name:  Howard M. Darmstadter
                         Title:  Assistant Secretary



                      CITIGROUP INC.



                      By: /s/ Marla A. Berman
                         --------------------------------------------
                         Name:  Marla A. Berman
                         Title:  Assistant Secretary




                                 Page 7
                               of 7 Pages


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                          EXHIBIT INDEX TO SCHEDULE 13G
                          -----------------------------



EXHIBIT 1
- ---------

Agreement between SSB Holdings and Citigroup as to joint filing of
Schedule 13G


EXHIBIT 2
- ---------

Disclaimer of beneficial ownership by SSB Holdings and Citigroup







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                                    EXHIBIT 1
                                    ---------

                  AGREEMENT AS TO JOINT FILING OF SCHEDULE 13G
                  --------------------------------------------

Each of the undersigned hereby affirms that it is individually eligible to use
Schedule 13G, and agrees that this Schedule 13G is filed on its behalf.

Date: January 19, 1999

                      SALOMON SMITH BARNEY HOLDINGS INC.



                      By: /s/ Howard M. Darmstadter
                         --------------------------------------------
                         Name:  Howard M. Darmstadter
                         Title:  Assistant Secretary



                      CITIGROUP INC.



                      By: /s/ Marla A. Berman
                         --------------------------------------------
                         Name:  Marla A. Berman
                         Title:  Assistant Secretary










<PAGE>


                                   EXHIBIT 2
                                   ---------

                       DISCLAIMER OF BENEFICIAL OWNERSHIP
                       ----------------------------------

Each of the undersigned disclaims beneficial ownership of the securities
referred to in the Schedule 13G to which this exhibit is attached, and the
filing of this Schedule 13G shall not be construed as an admission that any of
the undersigned is, for the purpose of Section 13(d) or 13(g) of the Securities
Exchange Act of 1934, as amended, the beneficial owner of any securities
covered by this Schedule 13G.


Date: January 19, 1999



                      SALOMON SMITH BARNEY HOLDINGS INC.



                      By: /s/ Howard M. Darmstadter
                         -----------------------------
                         Name:  Howard M. Darmstadter
                         Title:   Assistant Secretary


                      CITIGROUP INC.



                      By: /s/ Marla A. Berman
                         --------------------------------------------
                         Name:  Marla A. Berman
                         Title:  Assistant Secretary