SUMMARYSpecific suppressor cells were demonstrated in rats that had carried tolerated skin allografts for long periods of time after being rendered tolerant at birth. These suppressor cells were able to transfer tolerance to sublethally irradiated syngeneic recipients and to inhibit cytotoxic antibody production in normal syngeneic recipients. Suppressive activity of these cells was shown to be radiosensitive. The presence of suppressor cells in tolerant animals was attributable to neonatal tolerance induction and not to skin grafting of neonatally treated animals. In some cases spleen cells from tolerant animals transferred adoptively or induced permanent tolerance to skin grafts, which suggests a long-lasting active mechanism of tolerance.