Professor Jacques Miller's major discovery arose during investigations of the role of the thymus gland in the pathogensis of leukemia in mice. In 1961 he showed that neonatal thymectomy in mice had profound effects on the development of the immune response in later life, especially the component related to transplantation immunity. Subsequently he has developed this discovery with great skill. He showed that even in adult life the thymus is essential for the maintenance of an adequate pool of immunologically competent cells. Grafting of thymus tissue reversed the effects of thymectomy, at least in part by a humoral effect. He has also shown that the thymus exports lymphocytes into the peripheral recirculating pool, and that these collaborate with bone marrow-derived lymphocytes in antibody formation. These discoveries have opened up wide areas of research in several important fields of medical biology, tissue transplantation, immunological deficiency syndromes and control of incipient cancer by immunological surveillance.