Sodium pyridinethione, in vitro, possesses a significant inhibitory effect on human tumor cell strains, such as Hela, CNE-2 and Raji cell strain. In vivo, it has not inhibitory effect on the transplanted tumors, such as S180, LLC and EC, but significantly enhance the inhibitory effects of 5-Fu on S180.Sodium pyridinethione markedly decreases the weight of spleen and thymus, and serum hemolysins reaction induced by SRBC in mice and skin delayed type hypersensitivity reaction induced by DNCB in guinea pigs It also inhibits lyphocyte transformation induced by PHA. All these effects are shown clear dose-response relationship. Our experiments showed that sodium pyridinethion is a new antitumor and immuno-inhibitory compound.