<PAGE>
                                                                    EXHIBIT 4.15
                                AMENDMENT NO. 13


         THIS AMENDMENT NO. 13, dated as of December 17, 2001 (the "AMENDMENT")
relating to the Credit Agreement referenced below, by and among NATIONAL MEDICAL
CARE, INC., a Delaware corporation, certain subsidiaries and affiliates party to
the Credit Agreement and identified on the signature pages hereto, and BANK OF
AMERICA, N.A., (formerly known as NationsBank, N.A.), as Paying Agent for and on
behalf of the Lenders. Terms used but not otherwise defined shall have the
meanings provided in the Credit Agreement.

                               W I T N E S S E T H

         WHEREAS, a $2.5 billion credit facility has been extended to National
Medical Care, Inc. and certain subsidiaries and affiliates pursuant to the terms
of that Credit Agreement dated as of September 27, 1996 (as amended and
modified, the "CREDIT AGREEMENT") among National Medical Care, Inc., the other
Borrowers, Guarantors and the Lenders identified therein, and NationsBank, N.A.,
as Paying Agent;

         WHEREAS, the Company has requested the modification of certain
covenants and certain other changes to the Credit Agreement more fully set forth
herein;

         WHEREAS, the requested consents and modifications described herein
require the consent of the Required Lenders; and

         WHEREAS, the Required Lenders have consented to the requested
modifications on the terms and conditions set forth herein and have authorized
the Paying Agent to enter into this Amendment on their behalf to give effect to
this Amendment;

         NOW, THEREFORE, IN CONSIDERATION of these premises and other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereto agree as follows:

         1.       The Credit Agreement is amended and modified in the following
respects:

                  1.1      In Section 1.1, the definition of "Permitted
         Investments" is hereby amended by renumbering the last clause thereof
         clause (xxiii) and inserting the following immediately prior thereto as
         a new clause (xxii):

                           (xxii) Investments by Holdings or any Subsidiary of
                  Holdings in the Refinancing Securities.

                  1.2      In Section 7.9, the Consolidated Leverage Ratio
         covenant in subsection (b) is hereby amended to read as follows:

                           (b)      CONSOLIDATED LEVERAGE RATIO. There shall be
                  maintained as of the end of each fiscal quarter to occur
                  during the periods shown, a Consolidated Leverage Ratio of not
                  greater than:
<TABLE>
<S>                                                                         <C>

                           December 31, 2000 through December 30, 2002      3.25:1.00
                           December 31, 2002 and thereafter                 3.00:1.00
</TABLE>
<PAGE>
                  1.3      Clause (iii) of Section 8.1(d) is hereby deleted in
         its entirety and the following is substituted therefore as a new clause
         (iii):

                           "(iii) the total amount of all such Indebtedness
                  shall not exceed $300 million outstanding at any time."

                  1.4      Clause (b) of Section 8.9 is hereby amended by
         inserting the following proviso at the end thereof:

                  "; PROVIDED, that Holdings and its Subsidiaries may at any
                  time and from time to time prepay, repurchase, redeem, defease
                  or otherwise acquire or retire all or any of the Refinancing
                  Securities as long as no Default or Event of Default shall
                  have occurred and be continuing or shall be caused thereby."

                  1.5      Clauses (a) and (b) of Section 8.10 are hereby
         deleted in their entirety and the following substituted therefor as a
         new Section 8.10:

                  "Neither Holdings nor the Company will make or permit any
                  Restricted Payments, unless and to the extent that (i) no
                  Default or Event of Default shall exist on a Pro Forma Basis
                  after giving effect thereto, and (ii) the amount of Restricted
                  Payments shall not in any event exceed in any fiscal year (a)
                  with respect to Holdings, the amount set forth on Schedule
                  8.10, and (b) with respect to the Company, not more than 50%
                  of Consolidated Net Income of the Company for the prior fiscal
                  year; PROVIDED, that, without regard to the amounts set forth
                  above, Holdings and its Subsidiaries may purchase, repurchase,
                  redeem, defease or otherwise acquire or retire all or any of
                  the Refinancing Securities."

                  1.6      In Section 8.11, [Sale Leasebacks] is hereby amended
         by deleting the reference to "$200 million" in clause (y) thereof, and
         replacing it with a reference to "$300 million".

         2.       The effectiveness of this Amendment is subject to receipt by
the Paying Agent of the following:

                  (i)      copies of this Amendment executed by the Company and
         the other members of the Consolidated Group identified on the signature
         pages hereto,

                  (ii)     the consent of the Required Lenders; and

                  (iii)    an amendment fee in an amount equal to twelve and
         one-half basis points (0.125%) of the aggregate amount of Commitments
         held by the Lenders consenting to this Amendment for the ratable
         benefit of such consenting Lenders.

         3.       Except as modified hereby, all of the terms and provisions of
the Credit Agreement (and Exhibits and Schedules) remain in full force and
effect.

         4.       The Credit Parties hereby affirm (i) the representations and
warranties set out in Section 6 of the Credit Agreement are true and correct as
of the date hereof (except those which expressly relate to an earlier period)
and (ii) no Default or Event of Default presently exists.

         5.       The Company agrees to pay all reasonable costs and expenses of
the Paying Agent in connection with the preparation, execution and delivery of
this Amendment, including without limitation the reasonable fees and expenses of
Moore & Van Allen, PLLC.



                                       2

<PAGE>
         6.       This Amendment may be executed in any number of counterparts,
each of which when so executed and delivered shall be deemed an original and its
shall not be necessary in making proof of this Amendment to produce or account
for more than one such counterpart.

         7.       This Amendment, and the Credit Agreement as amended hereby,
shall be governed by and construed and interpreted in accordance with the laws
of the State of New York.

                  [remainder of page intentionally left blank]




















                                       3


<PAGE>

        IN WITNESS WHEREOF, each of the parties hereto has caused a counterpart
of this Amendment to be duly executed and delivered as of the date first above
written.


BORROWERS:                      NATIONAL MEDICAL CARE, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                FRESENIUS MEDICAL CARE AG

                                By       /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Dr. Emanuele Gatti
                                Title:

                                By       /s/ Roberto Fuste
                                  ----------------------------------------------
                                Name:    Roberto Fuste
                                Title:

                                NMC DO BRASIL LTDA.,
                                a Brazil corporation

                                By       /s/ Horst Radke      /s/ Armin Karch
                                  ----------------------------------------------
                                Name:    Horst Radtke / Armin Karch
                                 Title:

                                NATIONAL MEDICAL CARE OF SPAIN, S.A.,
                                a Spanish corporation

                                By       /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Dr. Emanuele Gatti
                                Title:

                                NATIONAL MEDICAL CARE OF TAIWAN, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer


                                NMC CENTRO MEDICO NACIONAL, LDA.,
                                a Portuguese corporation

                                By       /s/ Ricardo Da Silva    /s/ John Allen
                                  ----------------------------------------------
                                Name:    Ricardo Da Silva / John Allen
                                Title:
<PAGE>
                                FRESENIUS MEDICAL CARE ARGENTINA, S.A.,
                                as successor by merger to
                                NMC DE ARGENTINA, S.A.,
                                an Argentine corporation

                                By       /s/ Guido Yagupsky    /s/ Horst Radtke
                                  ----------------------------------------------
                                Name:    Dr. Guido Yagupsky / Horst Radtke
                                Title:

                                FRESENIUS USA, INC.,
                                a Massachusetts corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                FRESENIUS MEDICAL CARE DEUTSCHLAND GmbH,
                                a German corporation

                                By       /s/ Rolf Groos
                                  ----------------------------------------------
                                Name:    Rolf Groos
                                Title:

                                By       /s/ Norbert Weber
                                  ----------------------------------------------
                                Name:    Norbert Weber
                                Title:

                                FRESENIUS MEDICAL CARE GROUPE FRANCE
                                (formerly known as Fresenius Groupe France
                                 S.A.), a French corporation

                                By       /s/ Udo Werle     /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Udo Werle / Dr. Emanuele Gatti
                                Title:

                                FRESENIUS MEDICAL CARE HOLDING, S.p.A.,
                                an Italian corporation

                                By       /s/ Emanuele Gatti      /s/ Rolf Groos
                                  ----------------------------------------------
                                Name:    Dr. Emanuele Gatti / Rolf Groos
                                Title:

                                FRESENIUS MEDICAL CARE ESPANA S.A.,
                                a Spanish corporation

                                By       /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Dr. Emanuele Gatti
                                Title:

<PAGE>
                                FRESENIUS MEDICAL CARE MAGYAROSZA KfG,
                                a Hungarian corporation

                                By       /s/ Norman Erhard
                                  ------------------------------------------
                                Name:    Norman Erhard
                                Title:

                                BIO-MEDICAL APPLICATIONS OF ALABAMA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF FLORIDA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF GEORGIA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF INDIANA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF KENTUCKY, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF LOUISIANA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer


<PAGE>

                                BIO-MEDICAL APPLICATIONS OF MARYLAND, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF
                                  MASSACHUSETTS, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF
                                  NORTH CAROLINA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF OHIO, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF PENNSYLVANIA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF
                                  SOUTH CAROLINA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS OF TEXAS, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer




<PAGE>

                                BIO-MEDICAL APPLICATIONS OF VIRGINIA, INC.

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                LIFECHEM, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

GUARANTORS:                     FRESENIUS MEDICAL CARE HOLDINGS, INC.,
                                a New York corporation formerly known as WRG-NY

                                By       /s/ Jerry Schneider
                                  ----------------------------------------------
                                Name:
                                Title:

                                NATIONAL MEDICAL CARE, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                BIO-MEDICAL APPLICATIONS MANAGEMENT CO.,
                                INC., a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                FRESENIUS MEDICAL CARE AG,
                                a German corporation

                                By       /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Dr. Emanuele Gatti
                                Title:

                                By       /s/ Roberto Fuste
                                  ----------------------------------------------
                                Name:    Roberto Fuste
                                Title:


<PAGE>
                                FRESENIUS USA, INC.,
                                a Massachusetts corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                FRESENIUS MEDICAL CARE DEUTSCHLAND
                                GmbH, a German corporation

                                By       /s/ Rolf Groos
                                  ----------------------------------------------
                                Name:    Rolf Groos
                                Title:

                                By       /s/ Norbert Weber
                                  ------------------------------------------
                                Name:    Norbert Weber
                                Title:

                                FRESENIUS MEDICAL CARE GROUPE
                                FRANCE, a French corporation
                                (formerly known as Fresenius Groupe
                                France S.A.)

                                By       /s/ Udo Werle     /s/ Emanuele Gatti
                                  ----------------------------------------------
                                Name:    Udo Werle / Dr. Emanuele Gatti
                                Title:

                                FRESENIUS SECURITIES, INC.,
                                a California corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                NEOMEDICA, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                FMC FINANCE S.A.,
                                a Luxembourg corporation

                                By       /s/ John Allen
                                  ----------------------------------------------
                                Name:    John Allen
                                Title:



<PAGE>
                                FMC TRUST FINANCE S.a.r.l. LUXEMBOURG,
                                a Luxembourg corporation

                                By       /s/ Andrea Stopper
                                  ----------------------------------------------
                                Name:    Andrea Stopper
                                Title:

                                FMC TRUST FINANCE S.a.r.l. LUXEMBOURG III,
                                a Luxembourg corporation

                                By       /s/ Gabriele Dux
                                  ------------------------------------------
                                Name:    Gabriele Dux
                                Title:

                                QCI HOLDINGS, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer

                                SRC HOLDINGS, INC.,
                                a Delaware corporation

                                By       /s/ Marc Lieberman
                                  ----------------------------------------------
                                Name:    Marc Lieberman
                                Title:   Treasurer





<PAGE>


PAYING AGENT:                   BANK OF AMERICA, N.A. (formerly known as
                                NationsBank, N.A.), as Paying Agent
                                for and on behalf of the Lenders

                                By_________________________________
                                Name:
                                Title:

<PAGE>

                           CONSENT TO AMENDMENT NO. 13


Bank of America, N.A. (formerly known as NationsBank, N.A.),
  as Paying Agent
101 N. Tryon Street, 15th Floor
NC1-001-15-04
Charlotte, North Carolina 28255
Attn:  James D. Young, Agency Services

         Re:      Credit Agreement dated as of September 27, 1996 (as amended
                  and modified, the "CREDIT AGREEMENT") among National Medical
                  Care, Inc., the other Borrowers, Guarantors and Lenders
                  identified therein and NationsBank, N.A. (now known as Bank of
                  America, N.A.), as Paying Agent. Terms used but not otherwise
                  defined shall have the meanings provided in the Credit
                  Agreement.

                  Amendment No. 13 dated November __, 2001 (the "SUBJECT
                  AMENDMENT") relating to the Credit Agreement

Ladies and Gentlemen:

         This should serve to confirm our receipt of, and consent to, the
Subject Amendment. We hereby authorize and direct you, as Paying Agent for the
Lenders, to enter into the Subject Amendment on our behalf in accordance with
the terms of the Credit Agreement upon your receipt of such consent and
direction from the Required Lenders, and agree that Company and the other Credit
Parties may rely on such authorization.

                                          Sincerely,



                                          ______________________________________
                                                   [Name of Lender]

                                          By:___________________________________
                                          Name:
                                          Title: