In recent years, a detailed understanding of the cellular and molecular basis of the immune response has been achieved. These advances, coupled with the technology for producing monoclonal antibodies, have made possible to consider highly specific and potentially powerful methods of immunosuppressive treatment. This promise of potent and specific treatment has not been entirely fulfilled on clinical practice. A preclinical nonhuman primate model of renal transplantation is described. The model has been used to investigate two monoclonal anti-IL-2 receptor antibodies, one of which was found to be effective. Anti-Tac, an lgG2a mouse antihuman monoclonal antibody, prolongs graft survival in cynomolgus monkeys from 12 to 19 days. The role of such a model in bringing new monoclonal antibodies to the clinic is described.