Peritoneal carcinomatoses, an ordinary way for human colon carcinoma to spread, are incurable. When rat peritoneal carcinomatoses of colon origin were treated with endotoxins i.p. administered 3 days after the tumour cell injection, 70% of the BDIX rats were alive 30 weeks after the PROb tumour cell injection whereas all the untreated rats died of their tumour within 14 weeks. The study of the effector cells involved in the antitumour effect of endotoxins suggests that T lymphocytes are required for this effect. The endotoxin effect is local and is not reflected by the cytolytic activity of peritoneal cells.