Current experience with renal transplantation indicates that projected five-year survival rates greater than 90% can be attained in recipients receiving kidneys from related donors.1,2 This achievement, even more remarkable because the basic mechanisms of immunologic rejection remain obscure, has been gained with the sustained use of azathioprine and prednisone. These two drugs, introduced into practice in 1961,3 remain to date the primary agents of clinical immunosuppression. The initial use of azathioprine, at dosages of 10 mg/kg body weight, was attended by severe bone marrow depression and a 40% mortality from infection during the first three months.4 Long-term survival became possible only after this "high-risk" period was passed. The dosages could then be reduced to levels which maintained immunosuppression, but reduced the risk from infection. As various techniques and auxiliary drugs were developed which eliminated the need for such high doses of axathioprine in the immediate postoperative period,