EXHIBIT 4.19

                      Joint Venture Shareholders' Agreement

                                  May 16, 1996

Between:  York Medical Inc. ("YM"), a company incorporated under the laws of the
Province of Ontario,  Canada,  CBQYM Inc., a company incorporated under the laws
of Barbados  ("CBQYM")  and CIMAB S.A.  ("CIMAB") a Cuban  company  incorporated
under  the laws of the  Republic  of Cuba,  representing  Centro  de  Bioactivos
Quimicos of the Universidad Central de Las Villas ("CBQ").




                                    RECITALS

Whereas:

o     CIMAB entered into a licencing  agreement  with YM on behalf of CBQ, dated
      May 3, 1995 (the "Licencing Agreement");

o     CBQYM has been incorporated to serve as a joint venture company,  owned as
      to 80% of its  common  equity by YM and as to 20% by CIMAB.  to market the
      licenced  products  assigned  to  it  by  YM  pursuant  to  the  Licencing
      Agreement;

o     Certain of the benefits and obligations of the Licence Agreement have been
      assigned to CBQYM by YM; and,

o     The purpose of this agreement is to set out the  relationship  between the
      parties hereto so that they will proceed on a mutually beneficial basis.

Now,  therefore,  for good and valuable  consideration and the sum of one dollar
(Canadian  funds)  paid by each party to the  other,  receipt of which is hereby
acknowledged.


<PAGE>


o     CBQYM shall have an unlimited  number of common shares,  the initial issue
      of which shall be as to 80% to YM and as to 20% to CIMAB.

o     No issued and  outstanding  shares in the  capital of CBQYM shall be sold,
      transferred  or  otherwise  disposed of to any person  without the written
      consent of each of YM and CIMAB.

o     The issue of any  additional  common  shares in the capital of CBQYM shall
      first be to each of YM and CIMAB in proportion to their shareholdings, and
      thereafter  to any other  person,  but only with the consent of both of YM
      and CIMAB, such consent not to be withheld unreasonably.

o     The board of directors of CBQYM shall consist of nine directors,  three of
      whom shall be  nominees  of YM, two of whom shall be nominees of CIMAB and
      four of whom shall be a resident of Barbados, nominated by YM. The initial
      directors of CBQYM shall be as set out in Appendix A hereto.  The election
      of directors shall take place annually. The board is required to meet once
      per  year in the  Barbados,  and at  least  one  other  time per year at a
      location  to be  agreed  to  among  the  directors  or by  telephone,  and
      otherwise as often as deemed  necessary for the conduct of the business of
      the joint venture in accordance with the Articles of Incorporation and the
      by-law's of CBQYM.  The by laws of CBQYM shall  provide  that a quorum for
      meetings of the board of  directors  shall be a minimum of four  directors
      and must include one of each of the YM and CIMAB nominees, notice shall be
      a minimum of 30 days prior to any  meeting and  meetings  may be called at
      any time by two directors.

o     CBQYM shall use its best efforts to undertake and perform the terms of the
      assigned  Licence  Agreement.  In all events  CBQYM shall  operate  solely
      through  YM in  CBQYM's  commercialization  of  products  pursuant  to the
      Licence  Agreement,  any  assignment  thereof  and  regarding  any  future
      licencing agreements.


<PAGE>


o     All  material  and  out-of-the-ordinary-course  of business  contracts  of
      CBQYM,  including  contracts or  agreements  respecting  the  borrowing of
      money,  the issuance of guarantees,  the entering into of  non-arms-length
      agreements,  the  lending of money to any  persons  related to CBQYM,  the
      approval  of the  strategic  marketing  plan,  the  approval of the annual
      budget, the approval for acquisitions,  dispositions and amalgamations and
      the  pledging of property are required to be approved by a majority of the
      board of directors  including the approval of at least one nominee of each
      of YM and CIMAB.  Such approval may be evidenced either by a resolution of
      the  board of  directors  of  CBQYM or a  written  consent  signed  by the
      required  number of directors.  Any and all other matters  relating to the
      business  and  affairs  of CBQYM  shall be  governed  by the  Articles  of
      Incorporation  and  by-laws or, to the extent not  provided in them,  by a
      majority of the directors of CBQYM evidenced as provided above.

o     The parties  agree that all of the earnings of CBQYM shall be  distributed
      annually as dividends to the shareholders.  Any variation from this policy
      must he approved by a vote of the  majority  of the  directors,  including
      approval of all of the nominees of YM and CIMAB, respectively.

o     The  auditors  of  CBQYM  shall be KPMG  Peat  Marwick.  CBQYM's  full and
      accurate  records  of  the  net  revenues  shall  be  made  available  for
      inspection by CIMAB.

In witness  whereof,  the parties  hereto have executed  this  Agreement by duly
authorized officers or representatives thereof.

CIMAB S.A.                                             YORK MEDICAL INC.


duly authorized to sign                                duly authorized to sign



                         CBQYM Inc.

                         duly authorized to sign


<PAGE>



                                   APPENDIX A

                         Initial Directors of CBQYM Inc.

                                Hugh B. Anderson

                                 Diane J. Katina

                            Christopher M. Ostrovski

                               Cohn W. D'A Daniel

                                  Paul W. Haddy

                                 Allan B. Lewis

                                 Philip S. Young

                           Dr. Ervello Olazabal Manso

                           Ing. Humberto Martin Perez