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                                                                   Exhibit 10.23
 
                                  FRESENIUS AG
                              FRESENIUS USA, INC.
 
                                                                     May 8, 1996
 
W. R. Grace & Co.
One Town Center Road
Boca Raton, FL 33486-1010
 
Ladies and Gentlemen:
 
     We refer to the Agreement and Plan of Reorganization dated as of February
4, 1996 (the "Agreement") by and between W. R. Grace & Co. ("Grace") and
Fresenius AG ("Fresenius AG") and confirm our agreement as follows:
 
          1. In each of (a) clause (iii) of the proviso to paragraph (e) of
     Section 4.2 of the Agreement and (b) the second sentence of section 2.3(a)
     of Exhibit D to the Agreement, the figure "50.3%" shall be substituted for
     the figure "51%."
 
          2. Subject to the terms of paragraph 6 hereof, and on the condition
     that no more than 70 million Newco Ordinary Shares are outstanding
     immediately following consummation of the Reorganization, in accordance
     with Section 4.2(e) of the Agreement, Fresenius AG hereby notifies Grace
     that the Fresenius USA Consideration Per Share shall be equal to 0.37067735
     Newco Ordinary Shares.
 
          3. Subject to the terms of paragraph 6 hereof and Section 9.1 of the
     Agreement, in accordance with Section 9.13 of the Agreement, Fresenius USA
     hereby undertakes the obligations contained therein as a party and, for
     itself, makes directly to Grace the representations and warranties
     contained in the Agreement with respect to itself.
 
          4. Subject to the terms of Section 9.1 of the Agreement, Fresenius USA
     hereby represents and warrants to Grace that subject only to the receipt of
     the requisite approval of its shareholders, Fresenius USA has the requisite
     corporate power and authority and has taken all corporate action necessary
     in order to execute, deliver and perform each Transaction Agreement to
     which it is a party and to consummate the transactions contemplated by the
     Agreement and the Transaction Agreements including, without limitation, the
     approval of the Board of Directors of Fresenius USA and the resolution of
     the Board of Directors of Fresenius USA to recommend the transactions
     contemplated by the Agreement and the Transaction Agreements for approval
     by Fresenius USA shareholders, subject to their fiduciary duties.
 
          5. All notices, requests, claims, demands and other communications to
     Fresenius USA under the Agreement shall be in writing and shall be given
     (and shall be deemed to have been duly given upon receipt) by delivery in
     the manner set forth in the Agreement, addressed as follows:
 
           Fresenius USA, Inc.
           2637 Shadelands Drive
           Walnut Creek, CA 94598
           Attn: Dr. Ben Lipps
           Fax: (510) 988-1941
 
           with copies to:
 
           Ropes & Gray
           One International Place
           Boston, MA 02110-2624
           Attn: Winthrop G. Minot, Esq.
           Fax: (617) 951-7051
 
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                                                                   Exhibit 10.23

W. R. Grace & Co.
May 8, 1996
Page 2
 
           and
 
           O'Melveny & Myers
           Citicorp Center
           153 East 53rd Street
           New York, NY 10022-4611
           Attn: Dr. Ulrich Wagner
           Fax: (212) 326-2061
 
          6. If there shall be in excess of 9,253,331 Fresenius USA Common Share
     Equivalents outstanding immediately prior to the effective time of the
     Fresenius USA Merger, the terms of paragraphs 2, 3 and 4 hereof shall be
     automatically void and of no further force or effect, without any action on
     the part of any party hereto.
 
          7. Terms capitalized but not defined in this letter shall have the
     meanings ascribed to them in the Agreement.
 
          8. This letter agreement may be executed in several counterparts, each
     of which shall be an original and all of which shall be deemed to be one
     and the same Agreement.
 
          9. This letter agreement shall be governed by and construed in
     accordance with the laws of the State of New York.
 
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                                                                   Exhibit 10.23
 
     Please confirm your agreement to the foregoing by executing duplicate
copies of this letter and returning one executed copy to each of Fresenius AG
and Fresenius USA.
 
                                          Very truly yours,

 
                                          FRESENIUS AG

 
                                          By: /s/  GERD KRICK
                                            ------------------------------------
                                            Name: Gerd Krick
                                            Title: Chief Executive Officer

 
                                          By: /s/ MATHIAS KLINGLER
                                            ------------------------------------
                                            Name: Mathias Klingler
                                              Title: President and Chief
                                              Operating
                                                 Officer -- Dialysis Systems
                                              Division

 
                                          FRESENIUS USA, INC. (with
                                          respect only to paragraphs 3
                                          through 9, inclusive)

 
                                          By: /s/  BEN LIPPS
                                            ------------------------------------
                                            Name: Ben Lipps
                                            Title: President

 
AGREED
 
W. R. Grace & Co.

 
By: /s/  Paul McMahon
    ----------------------------------
    Name: Paul McMahon
    Title: Vice President
 
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