Background. Antibodies and fusion proteins specific for CD80, CD86, and CD154 have shown promise as agents capable of inducing donor-specific tolerance in rodents. These agents have also been shown to be synergistic with one another in many settings of counter-adaptive immunity. In the nonhuman primate, monoclonal antibodies specific for CD80 and CD86 have prolonged the time to rejection of renal allografts but have not resulted in tolerance. A monoclonal antibody specific for CD154 has resulted in markedly prolonged survival of kidney, islet, cardiac, and skin allografts, but again most animals have eventually developed rejection after prolonged periods of rejection-free survival off therapy. Methods. A combination of monoclonal antibodies specific for CD80, CD86, and CD154 were used in a mismatched nonhuman primate renal-allograft model. Doses used were based on optimized treatment protocols for each agent individually. Results. Treatment of four rhesus macaques with this combination yielded a mean rejection-free survival of 565 days (311-911 days), significantly greater than untreated controls (mean survival=7.0 days, P=0.001) and animals treated with only a combination of anti-CD80 and CD86 (mean survival=191 days, P=0.01). The survival of animals treated with this combination of monoclonal antibodies was not significantly greater than those treated with anti-CD154 alone, but the production of alloantibody was delayed compared with monotherapy anti-CD154. Conclusion. These data suggest that a synergy exists between these agents, particularly with regard to T-dependent B-cell responses, but that they fail to induce durable tolerance in nonhuman primates.