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                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

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

                                   SCHEDULE TO
                                 (Rule 14d-100)
          TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR 13(e)(1) OF
                       THE SECURITIES EXCHANGE ACT OF 1934
                                (Amendment No. 4)

                                   Genset S.A.
                            (Name of Subject Company)

                                  Serono S.A.,
                           Serono France Holding S.A.
                                       and
                                   Serono B.V.
                       (Name of Filing Persons (Offerors))

             Ordinary Shares, nominal value of (euro)3.00 per share
                                       and
   American Depositary Shares evidenced by American Depositary Receipts, each
     American Depositary Share representing one-third of one Ordinary Share
                         (Title of Class of Securities)
                                    37244T104
                      (CUSIP Number of Class of Securities)

                                Nathalie Joannes
                                 General Counsel
                                   Serono S.A.
                             15bis Chemin des Mines
                                 Case Postale 54
                                CH-1211 Geneva 20
                                   Switzerland
                               011-41-22-739-3113
                                    Copy to:

           Jean-Marie Ambrosi                        William A. Groll
   Cleary, Gottlieb, Steen & Hamilton       Cleary, Gottlieb, Steen & Hamilton
         41, Avenue de Friedland          City Place House, 55 Basinghall Street
           75008 Paris, France               London EC2V 5EH, United Kingdom
          011 33 1 40 74 68 00                     011 44 207 614 2200


                 (Name, Address and Telephone Numbers of Persons
  Authorized to Receive Notices and Communications on Behalf of Filing Persons)

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

                            Calculation of Filing Fee
--------------------------------------------------------------------------------
         Transaction Valuation*                   Amount of Filing Fee**
               $34,722,144                              $3,194.44
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*    Estimated solely for purposes of calculating the filing fee in accordance
     with Section 14(g)(3) of the Exchange Act. The Transaction Valuation was
     calculated on the basis of (i) the offer price of (a) Euro 3.25 for each of
     the 6,944,007 ADSs outstanding; (b) Euro 9.75 for each of the 300,352
     ordinary shares held by U.S. holders within the meaning of Rule 14d-1(d)
     under the Exchange Act and the 62,257 ordinary shares that would be issued
     to U.S. holders upon exercise of outstanding options that are, or will
     become, exercisable prior to the expiration of the offer; (c) Euro 102.64
     for each of the 86,695 OCEANEs held by U.S. holders, and (d) Euro 1.00 for
     each of the 9,000 warrants held by U.S. holders issued prior to June 2002
     and Euro 6.50 for each of the 15,000 warrants held by U.S. holders issued
     during June 2002 (which results in an aggregate value offered in this
     tender offer in euro of Euro 35,108,335) and (ii) an exchange rate (using
     the noon buying rate in New York City for cable transfers in euro as
     certified for customs purposes by the Federal Reserve Bank of New York on
     July 10, 2002) of U.S. $0.9890 for one euro. The number of Shares, options
     to purchase Shares, OCEANEs and warrants held by U.S. holders and the
     number of ADSs is based on the most current information provided to the
     bidder by Genset S.A. as of the commencement of this offer, as modified to
     reflect information provided by Genset subsequently about the number of
     OCEANEs and warrants held by U.S. holders.
**   The filing fee, calculated in accordance with Section 14(g)(3) of the
     Securities Exchange Act of 1934, as amended, is $92 for every $1,000,000 of
     the aggregate amount of cash proposed to be offered.
|X|  Check the box if any part of the fee is offset as provided by Rule
     0-11(a)(2) and identify the filing with which the offsetting fee was
     previously paid. Identify the previous filing by registration statement
     number, or the Form or Schedule and the date of its filing.

Amount Previously Paid: $2,326.00          Filing Party:  Serono S.A. and Serono
                                           France Holding S.A.
Form or Registration No.: Schedule TO      Date Filed:  July 16, 2002
Amount Previously Paid: $868.44            Filing Party:  Serono S.A., Serono
                                           France Holding S.A. and Serono B.V.
Form or Registration No.: Schedule TO,     Date Filed:  August 21, 2002
Amendment No. 1
                        --------------------------------

|_|  Check the box if the filing relates solely to preliminary communications
     made before the commencement of a tender offer.

     Check the appropriate boxes below to designate any transactions to which
     the statement relates:

     |X| third-party tender offer subject  |_| going-private transaction subject
         to Rule 14d-1.                        to Rule 13e-3.
     |_| issuer tender offer subject to    |_| amendment to Schedule 13D under
         Rule 13e-4.                           Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results
of the tender offer: |_|
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This Amendment No. 4 (this "Amendment") amends and supplements the Schedule TO
filed on July 16, 2002, as previously amended (the "Schedule TO"). All
capitalized terms used in this Amendment and not otherwise defined herein have
the meanings ascribed to such terms in the U.S. Offer to Purchase (previously
filed as Exhibit 99(a)(1) to the Schedule TO).

Item 11.   Additional Information

Item 11 of the Schedule TO is hereby amended and supplemented by adding the
following paragraph:

           "Genset issued a press release on October 24, 2002 which relates to
the tender offer. The full text of the press release is filed as Exhibit
99(a)(19) to this Amendment and is incorporated by reference in its entirety."

Item 12.   Exhibits

The following exhibits are added to Item 12 of the Schedule TO:

99(a)(19)    Press Release issued by Genset on September 24, 2002


                                    SIGNATURE

         After due 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: October 24, 2002

                                           SERONO S.A.
                                                  /s/ Jacques Theurillat
                                           -----------------------------------
                                                       (Signature)

                                                    Jacques Theurillat
                                           -----------------------------------
                                                     (Name and Title)

                                                 Director and Deputy CEO
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                                           SERONO FRANCE HOLDING S.A.
                                                  /s/ Jacques Theurillat
                                           -----------------------------------
                                                       (Signature)

                                                    Jacques Theurillat
                                           -----------------------------------
                                                     (Name and Title)

                                                         Director
                                           -----------------------------------


                                           SERONO B.V.
                                                  /s/ Jacques Theurillat
                                           -----------------------------------
                                                       (Signature)

                                                    Jacques Theurillat
                                           -----------------------------------
                                                     (Name and Title)

                                                         Director
                                           -----------------------------------


                                                    /s/ Paul Wilkinson
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                                                       (Signature)

                                                      Paul Wilkinson
                                           -----------------------------------
                                                     (Name and Title)

                                                Authorized Representative
                                           -----------------------------------