SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549

                                 SCHEDULE 13G


                                 (Rule 13d-102)


             INFORMATION STATEMENT PURSUANT TO RULES 13d-1 AND 13d-2


                   Under the Securities Exchange Act of 1934
                          (Amendment No.   )

                             IDEC PHARMACEUTICALS CORP / DE
             -----------------------------------------------------
                                (Name of Issuer)

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

                                    449370105
             -----------------------------------------------------
                                 (CUSIP Number)



Check the following box if a fee is  being paid with this statement [ ].


- ----------
*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).



CUSIP No. 449370105                    13G                    Page 2  of 6 Pages



- --------------------------------------------------------------------------------
   1.  NAME OF REPORTING PERSON(S)
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON(S)

          Morgan  Stanley Dean  Witter &  Co.
          IRS  #  39-314-5972



- --------------------------------------------------------------------------------
   2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                             (a)  [  ]
                                                             (b)  [  ]


- --------------------------------------------------------------------------------
   3.  SEC USE ONLY




- --------------------------------------------------------------------------------
   4.  CITIZENSHIP OR PLACE OF ORGANIZATION

          The state of organization is Delaware.


- --------------------------------------------------------------------------------
   NUMBER OF      5.   SOLE VOTING POWER
     SHARES            0
  BENEFICIALLY    --------------------------------------------------------------
    OWNED BY      6.   SHARED VOTING POWER
      EACH             3,404,598
   REPORTING      --------------------------------------------------------------
     PERSON       7.   SOLE DISPOSITIVE POWER
      WITH             0
                  --------------------------------------------------------------
                  8.   SHARED DISPOSITIVE POWER
                       3,417,583
- --------------------------------------------------------------------------------
   9.  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          3,417,583

- --------------------------------------------------------------------------------
  10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*



- --------------------------------------------------------------------------------
  11.  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

          7.68%

- --------------------------------------------------------------------------------
  12.  TYPE OF REPORTING PERSON*

          IA, CO

- --------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


CUSIP No. 449370105                   13G                   Page 3  of 6 Pages





Item 1.     (a)   Name of Issuer:
                  IDEC PHARMACEUTICALS CORP / DE
                 --------------------------------------------------------------
            (b)   Address of Issuer's Principal Executive Offices:
                  11011 TORREYANA ROAD
                  SAN DIEGO
                  CA 92121
                  --------------------------------------------------------------
Item 2.     (a)   Name of Person Filing:
                  (a)  Morgan Stanley Dean Witter & Co.
                  (b)  Van Kampen Asset Management Inc.

                  --------------------------------------------------------------
            (b)   Address of Principal Business Office, or if None, Residence:
                  (a)  1585 Broadway
                       New  York, New  York  10036

                  (b)  One Parkview Plaza
                       Oakbrook Terrace, IL 60181

                  --------------------------------------------------------------
            (c)   Citizenship:
                           Incorporated  by  reference  to  Item  4  of  the
                           cover page  pertaining  to  each  reporting  person.

                               -------------------------------------------------
            (d)   Title of Class of Securities:
                           Common Stock
                  --------------------------------------------------------------
            (e)   CUSIP Number:
                           449370105
                               -------------------------------------------------

Item 3.       (a)   Morgan  Stanley  Dean  Witter  & Co.  is (e)  an  Investment
                    Adviser  registered  under  Section  203 of  the  Investment
                    Advisers Act of 1940.






CUSIP No. 449370105                   13-G                   Page 4  of  6 Pages



Item 4.     Ownership.

               Incorporated  by  reference  to  Items  (5) - (9) and (11) of the
               cover page.

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


                Inapplicable.

Item 6.     Ownership of More Than Five Percent on Behalf of Another Person.
               Accounts managed on a discretionary basis by Morgan Stanley Dean
               Witter & Co.  are known to have the right to receive or the power
               to direct the receipt of dividends from, or the proceeds from the
               sale of  such  securities.  No such account holds more than 5
               percent of the class.




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


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


Item 9.     Notice of Dissolution of Group.


Item 10.    Certification.

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


CUSIP No. 449370105                    13-G                   Page 5  of 6 Pages


                                  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.


Date:       February , 2001


Signature:  /s/ Dennine Bullard
            -----------------------------------------------------------------

Name/Title  Dennine Bullard / Morgan Stanley & Co. Incorporated
            -----------------------------------------------------------------
            MORGAN STANLEY DEAN WITTER & CO.



                       INDEX TO EXHIBITS                           PAGE
                       -----------------                           ----


EXHIBIT 2       Secretary's Certificate Authorizing Dennine Bullard    6
                to Sign on behalf of Morgan Stanley Dean Witter & Co.





* Attention. Intentional misstatements or omissions of fact constitute federal
  criminal violations (see 18 U.S.C. 1001).

(022597DTI)





                                                             Page 6 of 6 Pages
                                  EXHIBIT 1                   ------------------

                 MORGAN STANLEY, DEAN WITTER, DISCOVER & CO.

                           SECRETARY'S CERTIFICATE




        I,  Charlene R.  Herzer,  a duly  elected  and Assistant  Secretary
         of Morgan  Stanley  Dean Witter & Co., a corporation  organized and
         existing  under the laws of the State of Delaware  (the
         "Corporation"),  certify that Stuart J.M.  Breslow, Robert G. Koppenol
         and Dennine  Bullard are authorized to sign reports to be filed under
         Sections 13 and 16 of the Securities  Exchange Act of 1934 (the "Act")
         pursuant to the following:
             (1)      On May 31,  1997,  the Board of  Directors  of the
                      Corporation  granted  authority  to Stuart J.M.
                      Breslow  and Robert G.  Koppenol  to sign  reports to be
                      filed  under  Sections 13 and 16 of the Act on behalf of
                      the Corporation.

             (2)      On February 23, 2000, Donald G. Kempf,  Jr., the Executive
                      Vice President,  Chief Legal Officer and Secretary of the
                      Corporation,  delegated  authority to Dennine Bullard to
                      sign reports to be filed under Sections 13 and 16 of the
                      Act on behalf of the  Corporation.  Pursuant  to  Section
                      7.01 of the  Bylaws of the  Corporation  and resolutions
                      approved by the Board of Directors of the Corporation on
                      September 25,  1998, the Chief Legal Officer is authorized
                      to enter into  agreements and other  instruments on behalf
                      of the Corporation and may delegate such powers to others
                      under his jurisdiction.

          IN WITNESS  WHEREOF,  I have hereunto set my name and affixed the seal
          of the Corporation as of the 1st day of March, 2000.

                                                ______________________________
                                                Charlene R. Herzer
                                                Assistant Secretary