Spleens from Lewis X BN F1 hybrid rats were directly anastamosed to the circulation of parental strain Lewis rats by a microvascular technique; no immunosuppressive drugs were given. Donor spleens enlarged massively during the first month after implantation, then grew gradually smaller over succeeding months without compromising their blood supply. Renal allografts were placed into bilaterally nephrectomised recipients that had already borne a splenic allograft for three months. The grafts functioned perfectly and showed no signs of rejection five months later. Donor LBN skin allografts transplanted to these animals showed markedly prolonged survival while third-party BUF rat skin allografts were rejected in first-set fashion. High titres of anti-LBN haemagglutinating antibody could be detected in the circulation of spleen-grafted animals during the first month; the titres fell to zero by the fourth month. Cellular reactivity of recipient cells against LBN antigens measured by the popliteal node GVH assay fell to minimal levels by the third month. These data indicate that the immune response to allografted tissues may be specifically and markedly suppressed in adult animals by continued presence of a donor-strain spleen.