Human renal homotransplantation has been attempted since 1936.1 In 1956 Merrill et al2 reported the first successful renal homotransplantation, an isotransplantation between identical twins. This success demonstrated that technical problems of transplantation need not be a barrier if immunologic aspects could be brought under control. Attempts at transplantation were then accelerated, and various methods were introduced in an attempt to modify the rejection reaction. Promising results were obtained using sublethal total body irradiation,3 but difficulties have led to its abandonment by all but a few investigators.4 Chemical immunosuppression was introduced after demonstration by Schwartz and Dameshek that mercaptopurine (6-mercaptopurine) could impair the immunologic response.5 Azathioprine (Imuran, BW-57-322), a derivative of mercaptopurine, is now the most widely used drug for this purpose. Because of its bone marrow toxicity, relatively small maintenance doses are used. Azathioprine is augmented by actinomycin C and large doses of prednisone when