The results of a number of investigations indicate that one feature of advanced renal failure is impaired ability to reject allogeneous tissue grafts. In 1955, Hume, Merrill, Miller, and Thorn reported the transplantation of kidneys from unrelated donors to nine patients with terminal renal failure (1). Four of the nine kidneys functioned for 5 to 25 weeks even though the therapy given to prevent graft rejection was inadequate by current standards. In a study of six uremic patients, Dammin, Couch, and Murray reported histological evidence of skin allograft survival for as long as 115 days (2). In view of these reports, and because a large number of patients were being evaluated for kidney transplantation, we have undertaken a detailed study of the effect of irreversible kidney failure on a number of parameters of immunological capability. Our earlier studies (3) indicated that chronically uremic patients have a suppression of immediate and delayed cutaneous hypersensitivity.Many features of delayed hypersensitivity are also characteristics of the allograft rejection phenomenon. In both reactions, histologic examination reveals a predominantly lymphocytic infiltration. The two responses can be passively transferred by immunologically competent cells, but serum from an immune animal will not sensitize a nonimmune recipient. Extracts of human peripheral blood leukocytes transfer delayed hypersen-