MOUSE lymphocytes can be induced in vitro by treatment with phytohaemagglutinin (PHA), or with rabbit anti-mouse serum, and by the mixed lymphocyte reaction, to produce material(s) which is toxic to L cells1. The term lymphotoxins (previously termed lymphocyte cytotoxic factor) is applied here to the toxic factor(s). We report that lymphocytes from several animal species, including man, can be stimulated with phytohaemagglutinin in vitro to release lymphotoxin-like materials. © 1968 Nature Publishing Group.