Recent studies have suggested that the immunologic unresponsiveness of UV-irradiated mice against UV-induced fibrosarcomas might be due to the presence of suppressor lymphoid cells. In these experiments, we present additional evidence that suppressor lymphoid cells are present in lymph nodes and spleens of UV-irradiated mice and demonstrate that these cells are enriched after incubation on nylon wool columns, that they are inactive after treatment with anti-theta serum and complement, and that they are effective for at least 7 weeks after transfer to lethally x-irradiated mice. Splenectomy of UV-treated mice before tumor challenge did not restore their anti-tumor reactivity. The UV-induced suppressor cells appear to be specific for syngeneic UV-induced tumors, since they did not suppress the rejection of an allogeneic UV-induced tumor or two chemically induced syngeneic tumors.