The field of heterotransplantation is now being explored because of success with immunosuppressive measures in homotransplantation, and because of the scarcity of suitable human organs. Two patients in terminal uremia, maintained on dialysis, received heterotransplants from nonhuman primates. In the first case a renal heterotransplant from a rhesus monkey implanted in a 32-year old woman showed satisfactory immediate function but was removed after 10 days because of inadequate function. The second patient received a renal hetero transplant from a chimpanzee. A threatened rejection was reversed with immunosuppressive measures, but 2 months after transplantation the patient died of pneumonia. The transplanted kidneys showed no evidence of rejection.