Successful induction of tolerance to heart and kidney allografts has been obtained in rats. This is accomplished by lethally irradiating the recipient and delaying the repopulation 2 days. The frequency of takes is influenced by the radiation dose and type of hemopoietic cells used to repopulate the host. We are investigating the mechanism of the induction of tolerance in vivo and in vitro. Tolerant host rats lack the ability to respond to donor type antigens in a mixed lymphocyte and cell mediated lympholysis assays but they do respond to third party antigens in these same assays. Tolerance can be passively transferred in vivo by spleen cells from tolerant recipients but not by serum from the tolerant rats. Experiments are under way to determine the type of cell capable of transferring tolerance in vivo.