<PAGE>   1

                                                                   Exhibit 10.21


                             SUBORDINATED LOAN NOTE



$400,000,000                                                 AS OF MAY 18, 1999



     FOR VALUE RECEIVED, National Medical Care, Inc., a Delaware corporation
("NMC"), Lifechem, Inc., Bio-Medical Applications of Alabama, Inc., a Delaware
corporation, Bio-Medical Applications of Connecticut, Inc., a Delaware
corporation, Bio-Medical Applications of Fayetteville, Inc., a Delaware
corporation, Bio-Medical Applications of Florida, Inc., a Delaware corporation,
Bio-Medical Applications of Georgia, Inc., a Delaware corporation, Bio-Medical
Applications of Indiana, Inc., a Delaware corporation, Bio-Medical Applications
of Jersey City, Inc., a Delaware corporation, Bio-Medical Applications of
Kentucky, Inc., a Delaware corporation, Bio-Medical Applications of Louisiana,
Inc., a Delaware corporation, Bio-Medical Applications of Maryland, Inc., a
Delaware corporation, Bio-Medical Applications of Massachusetts, Inc., a
Delaware corporation, Bio-Medical Applications of Mississippi, Inc., a Delaware
corporation, Bio-Medical Applications of Missouri, Inc., a Delaware corporation,
Bio-Medical Applications of New Jersey, Inc., a Delaware corporation,
Bio-Medical Applications of North Carolina, Inc., a Delaware corporation,
Bio-Medical Applications of Ohio, Inc., a Delaware corporation, Bio-Medical
Applications of Oklahoma, Inc., a Delaware corporation, Bio-Medical Applications
of Pennsylvania, Inc., a Delaware corporation, Bio-Medical Applications of South
Carolina, Inc., a Delaware corporation, Bio-Medical Applications of Tennessee,
Inc., a Delaware corporation, Bio-Medical Applications of Texas, Inc., a
Delaware corporation, and Bio-Medical Applications of Virginia, Inc., a Delaware
corporation, Bio-Medical Applications of Wisconsin, Inc., a Delaware
corporation, (collectively, the "BORROWERS") jointly and severally promise to
pay to the order of Fresenius AG, a German corporation, or its specified
subsidiary, (the "LENDER") the lesser of (i) the principal amount of
$400,000,000 (Four Hundred Million Dollars), or (ii) the unpaid principal amount
of all Advances (as defined in Section 2) made by the Lender to the Borrowers
hereunder, together with interest accrued thereon at the rate set forth below,
on the date specified for repayment of such Advance pursuant to Clause 3 hereof
or such earlier date as such amounts may become payable pursuant to the terms
hereof.

     1.   The following terms used in this Note shall have the following
          meanings:

     "NMC CREDIT AGREEMENT" means that certain Credit Agreement dated as of
September 27, 1996, by and among NMC and certain of its subsidiaries and
affiliates, the Lenders identified therein, and the Paying Agent and Managing
Agents identified therein, as amended through the date hereof, as it may be
further amended, restated, supplemented or otherwise modified from time to time.

     "FMC" means Fresenius Medical Care AG, a German corporation.

     "9% NOTES" means the 9% Senior Subordinated Notes due 2006 of FMC and
assumed by FMC Trust Finance S.a.r.l. Luxembourg, issued pursuant to that
certain Senior Subordinated Indenture dated as of November 27, 1996 by and among
FMC Trust Finance S.a.r.l. Luxembourg, as issuer, Fleet National Bank (as
predecessor to State Street Bank and Trust Company), as trustee, and the
Subsidiary Guarantors named therein, as guarantors, as supplemented through the
date hereof, as it may be further amended, restated, supplemented or otherwise
modified from time to time.

     "7 7/8% NOTES" means the 7 7/8% Senior Subordinated Notes due 2008 of FMC
issued pursuant to that certain Senior Subordinated Indenture dated as of
February 19, 1998 by and among FMC Trust Finance S.a.r.l. Luxembourg, as issuer,
State Street Bank and Trust Company), as trustee, and the Guarantors named
therein, as guarantors, as it may be further amended, restated, supplemented or
otherwise modified from time to time.

     "7 3/8% NOTES" means the 7 3/8% Senior Subordinated Notes due 2008 of FMC
issued pursuant to that certain Senior Subordinated Indenture dated as of
February 19, 1998 by and among FMC Trust Finance


<PAGE>   2

S.a.r.l. Luxembourg, as issuer, State Street Bank and Trust Company), as
trustee, and the Guarantors named therein, as guarantors, as it may be further
amended, restated, supplemented or otherwise modified from time to time.

     All other capitalized terms used but not otherwise defined herein shall
bear the meanings assigned thereto in the NMC Credit Agreement.

     2.   The Lender may lend (but shall not have any commitment to lend) one or
more advances (each an "ADVANCE") to the Borrowers jointly and severally from
time to time upon request during the period from the date hereof to but
excluding September 30, 2003 in an aggregate amount which shall not exceed
$400,000,000. Amounts borrowed hereunder may be repaid and reborrowed. The
Lender shall have no obligation to make any Advance requested hereunder.

     3.   Each Advance shall be repaid in full on the date that is one, two or
three months after the date on which it is made, as agreed by the Borrowers and
the Lender on the date such Advance is made, or any other period agreed between
the Borrowers and the Lender; PROVIDED, that if no maturity date is so agreed,
such Advance shall have a term of one month.

     4.   The unpaid principal amount of each Advance made hereunder shall bear
interest at a fluctuating rate per annum equal to the Eurocurrency Rate (as
defined in and calculated pursuant to the NMC Credit Agreement) for an Interest
Period equivalent to the term of such Advance plus a margin, determined pursuant
to the pricing matrix set forth below, that is based on the Consolidated
Leverage Ratio (as defined in and calculated pursuant to the NMC Credit
Agreement), and shall change as and when the Applicable Percentage (as defined
in and calculated pursuant to the NMC Credit Agreement) changes:

    Pricing Level     Consolidated
                      Leverage Ratio                        Margin
    ----------------- ---------------------------------------------

    I                 <1.75                                 0.350%
    II                >1.75 but < 2.0                       0.375%
                      -
    III               >2.0 but < 2.5                        0.500%
                      -
    IV                >2.5 but < 3.0                        0.700%
                      -
    V                 >3.0 but < 3.25                       0.800%
                      -
    VI                >3.25 but < 3.75                      0.950%
                      -
    VII               >3.75 but < 4.0                       1.150%
                      -
    VIII              >4.0 but < 4.25                       1.350%
                      -
    IX                >4.25                                 1.600%
                      -

Interest shall be payable in arrears upon maturity, on any prepayment and on any
acceleration of the principal amount hereof and shall be computed on the basis
of a 360-day year for the actual number of days elapsed (including the first day
and excluding the last day).

     5.   Whenever any payment on this Note shall be stated to be due on a day
which is not a Business Day or is a day on which commercial banks are authorized
or required by law to close in the Federal Republic of Germany, such payment
shall be made on the next succeeding Business Day on which commercial banks are
not authorized or required by law to close in the Federal Republic of Germany,
and such extension of time shall be included in the computation of the payment
of interest on this Note.

     6.   All payments of principal and interest in respect of this Note shall
be made in lawful money of the United States in same day funds to the Lender's
Dollar account no: xxxxxxxxxxxx with Dresdner Bank Bad Homburg v.d.H., bank
code: 50080000, SWIFT code: DRESDEFF.

     7.   THE BORROWERS HEREBY COVENANT AND AGREE, AND THE LENDER, BY ITS
ACCEPTANCE HEREOF, HEREBY COVENANTS AND AGREES, THAT, TO THE EXTENT AND IN THE
MANNER HEREINAFTER SET FORTH THE PAYMENT OF THE PRINCIPAL OF THE INDEBTEDNESS
EVIDENCED HEREBY AND ANY INTEREST PAYABLE IN RESPECT THEREOF


<PAGE>   3


ARE HEREBY EXPRESSLY MADE SUBORDINATE AND SUBJECT IN RIGHT OF PAYMENT TO THE
PRIOR PAYMENT IN FULL OF ALL AMOUNTS THEN DUE AND PAYABLE IN RESPECT OF (I) ALL
OBLIGATIONS OF THE BORROWERS UNDER THE NMC CREDIT AGREEMENT, AND (II) IF ANY
BORROWER SHALL GUARANTY THE 9% NOTES, THE 7 7/8% NOTES OR THE 7 3/8% NOTES, ALL
"SENIOR INDEBTEDNESS" OF SUCH BORROWER (AS SUCH TERM IS DEFINED IN THE
INDENTURES PURSUANT TO WHICH SUCH NOTES ARE ISSUED) (COLLECTIVELY, THE
"PREFERRED INDEBTEDNESS").

     8.   It is hereby further specifically provided that to the extent (and
only to the extent) required by the Indentures pursuant to which the Notes were
issued, the indebtedness evidenced hereby shall rank pari passu with the 9%
Notes, the 7 7/8% Notes and the 7 3/8% Notes in right of payment and the
obligations (if any) of the Borrowers in respect thereof; provided, however,
that this provision shall not affect the relative rights (if any) of the holders
of the Notes against the Borrowers other than their rights in relation to the
Lender hereunder.

     9.   If a payment or distribution is made to the Lender in respect of this
Note that, in accordance with Clause 7 above, should not have been made, the
Lender agrees that it shall hold such payment or distribution in trust for the
holders of the Preferred Indebtedness and pay such payment or distribution over
to such holders of Preferred Indebtedness as their interests may appear.

     10.  If any Bankruptcy Event shall occur with respect to the Borrowers, all
amounts of principal and accrued interest outstanding under this Note shall
become immediately due and payable.

     11.  The Lender agrees, by its acceptance hereof, that before disposing of
this Note or any part hereof it will make a notation hereon of all Advances, the
maturity date of each such Advance and principal payments previously made
hereunder and of the date to which interest hereon has been paid; provided,
however, that the failure to make a notation of any Advance or any payment made
on this Note shall not limit or otherwise affect the obligation of the Borrowers
hereunder with respect to payments of principal or interest on this Note.

     12.  THIS NOTE AND THE OBLIGATIONS OF THE BORROWERS ARISING HEREUNDER AND
ALL OTHER ASPECTS HEREOF SHALL BE DEEMED TO BE MADE UNDER, SHALL BE GOVERNED BY,
AND SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE LAWS OF NEW YORK
WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES.

     13.  The obligations of the Borrowers arising under this Note may be
prepaid in whole or in part, together with all accrued interest thereon, without
penalty or premium with the Net Proceeds of any Equity Transaction, or with the
prior consent of the Lender.

     14.  The terms of this Note are subject to amendment only by a writing
signed by the Borrowers and the Lender.

     15.  In no event shall any interest be payable under this Note to the
extent that the payment thereof would be prohibited by applicable law.

     16.  The Borrowers hereby waives diligence, presentment, protest, demand
and notice of every kind and, to the full extent permitted by law, the right to
plead any statute of limitations as a defense to any demand hereunder.

     17.  No delay on the part of the Lender in the exercise of any right or
remedy shall operate as a waiver thereof, and no single or partial exercise by
the Lender, of any right or remedy shall preclude any other or further exercise
of any other right or remedy.

     18.  In case any provision in or obligation under this Note shall be
invalid, illegal or unenforceable in any jurisdiction, the validity, legality
and enforceability of the remaining provisions or obligations, or of such
provision or obligation in any other jurisdiction, shall not in any way be
affected or impaired thereby.


<PAGE>   4


     IN WITNESS WHEREOF, this Note has been executed as of the day and year and
at the place first written above.

                                           NATIONAL MEDICAL CARE, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           LIFECHEM, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF ALABAMA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           CONNECTICUT, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           FAYETTEVILLE, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer


<PAGE>   5

                                           BIO-MEDICAL APPLICATIONS OF FLORIDA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF GEORGIA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF INDIANA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF JERSEY
                                           CITY, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF KENTUCKY,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer

<PAGE>   6

                                           BIO-MEDICAL APPLICATIONS OF
                                           LOUISIANA, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF MARYLAND,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           MASSACHUSETTS, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           MISSISSIPPI, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer



                                           BIO-MEDICAL APPLICATIONS OF MISSOURI,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer


<PAGE>   7


                                           BIO-MEDICAL APPLICATIONS OF NEW
                                           JERSEY, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF NORTH
                                           CAROLINA, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF OHIO,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF OKLAHOMA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           PENNSYLVANIA, INC.


                                           By: /s/  Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer

<PAGE>   8


                                           BIO-MEDICAL APPLICATIONS OF SOUTH
                                           CAROLINA, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF TEXAS,
                                           INC.


                                           By: /s/  Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF VIRGINIA,
                                           INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




                                           BIO-MEDICAL APPLICATIONS OF
                                           WISCONSIN, INC.


                                           By: /s/ Marc S. Lieberman
                                           -------------------------------------
                                               Name:  Marc S. Lieberman
                                               Title:    Assistant Treasurer




ACKNOWLEDGED AND AGREED:


FRESENIUS AG


By:     /s/  Dr. K.-D. Schwab                       /s/ Dr. D. Blumenhagen
----------------------------------------------------------------------------
      Name:  Dr. K.-D. Schwab                       Dr. D. Blumenhagen
      Title:    SVP Finance                         Treasurer



<PAGE>   9


                         TRANSACTIONS ON PROMISSORY NOTE


<TABLE>
<CAPTION>

                 Amount of         Maturity Date    Amount of         Amount of        Outstanding
                 Advance Made      of Such          Principal Paid    Interest Paid    Principal Balance   Notation Made
Date             This Date         Advance          This Date         This Date        This Date           By
----             ------------      -------------    --------------    -------------    -----------------   -------------
<S>              <C>               <C>              <C>               <C>              <C>                 <C>


</TABLE>