Sprague--Dawley rats of both sexes received 2 mg/kg body wt (group 1) or 0.4 mg/kg body wt (group 2) of 2-[3-(2-chloroethyl)-3-nitrosoureido]-D-glucopyranose (chlorozotocin) intraperitoneally in weekly injections for life. A dose-dependent carcinogenic effect of chlorozotocin was found (animals with malignant tumors in group 1: males 88%, females 89%, in group 2: males 83%, females 63%, in controls: males 10%, females 24%). Chlorozotocin reduced the life expectancy of treated animals significantly, compared to solvent controls. Histologically, the chlorozotocin-induced tumors were shown to be mesotheliomas or sarcomas of the peritoneal cavity (undifferentiated type, fibrosarcomas, myosarcomas).