Studies in non-human primates to evaluate tolerance strategies in organ transplantation have led to innovation in human transplantation. The two strategies we have studied in detail in non-human primates are T-cell depletion by anti-CD3 immunotoxin and co-stimulation blockade. Each of these strategies has been extended into early human trials in renal transplantation. The results of these human and non-human primate studies are summarized. Continued progress in better and safer immunosuppressive methods remains closely linked to research using non-human primates. However, there has not been a one-to-one correspondence between efficacy in the primate and efficacy in humans. Rather, principles can be derived from non-human primate studies that can be extended into human trials with the knowledge that regimens will likely differ in humans compared to non-human primates.