We have demonstrated that DA hearts grafted into PVG rats were completely protected from rejection by simultaneous liver transplantation from the same donor. In subsequent experiments PVG animals were given DA hearts followed 5 or 6 days later by livers from the same donor strain. Instead of the expected rapid rejection, all the grafts survived for at least 18 days, despite showing definite graft-swelling, a reliable clinical sign of early rejection, immediately prior to liver transplantation. In all 13 rats the heart size returned to normal and a strong beat returned within a few days. 6 animals survived indefinitely with healthy, beating heart grafts and the remaining 7 animals died of liver transplant rejection, but in these animals also the hearts were beating normally immediately prior to death. Histological examination of the hearts revealed no active rejection, but there was extensive myocardial scarring, compatible with resolution of a rejection reaction. It seems, therefore, that the liver grafts had entirely absorbed the vigorous immune response, terminating that which had already begun in the heart. This immunosuppressive effect was donor-specific and far more powerful than that of the immunosuppressive agent cyclosporine.