INTRODUCTION In organ transplantation the immunosuppressive drugs have dramatically improved the life of transplant recipients over the past 30 years, but two notable problems remain:drug-associated toxicities, and failure of these drugs to prevent chronic graft dysfunction and extend long-term graft survival. Given these circumstances, the scientific discussion is dominated by designing effective ways to induce transplantation tolerance. 'Transplantation tolerance' describes a state in which a donor is 'accepted' without chronic immunosuppressive therapy, while the remainder of the immune system is left intact. Thus, lack of a pathogenic response to the alloantigen is specific, and the recipient is capable of responding to potentially pathogenic microorganisms and malignancies. There is abundant evidence for active immunoregulatory mechanisms, which may operate to maintain transplant tolerance.