Monoclonal antibody technology has become an important diagnostic and therapeutic tool in many areas of medicine, including the prevention and treatment of organ transplant rejection. The monoclonal antibody that has been most widely used clinically is OKT3, a murine monoclonal antibody directed against the CD3 receptor on the surface of human T cells. In heart transplant recipients OKT3 successfully reverses rejection refractory to conventional immunosuppression. Monoclonal antibodies that are partially of human origin reduce the problem of sensitization in human recipients. The production of a fully human Fab' that can be used in place of the whole monoclonal antibody is being actively investigated. Strategies designed to induce tolerance to the allograft will involve the combined administration of monoclonal antibodies and donor antigens.