A very high incidence of spontaneous hepatomas occurs in male mice of our 3 substrains of strain C3H fed the usual diet of NCI pellets. The incidence in C3H males was 85 percent, in C3Hf males 72 percent, and in C3He males 78 percent. Intraperitoneal injection of 20-mg. urethan increased the occurrence of hepatomas in males of all 3 substrains. The occurrence of hepatomas in males that received 8 injections of 20-mg. urethan each was also greater than that in the uninfected controls, though in this study the occurrence was not shown to be greater than that in the males receiving 1 injection. Other lesions induced or potentiated by the urethan included hemorrhagic lesions of the liver, some of which appeared to be neoplastic, and pulmonary tumors. That the C3H males had the mammary-tumor agent, whereas the C3Hf and C3He males did not, had no influence on the occurrence of the hepatomas, the pulmonary tumors, or the hemorrhagic lesions of the liver. C3Hf males fed Purina laboratory chow had a 57 percent incidence of spontaneous hepatomas.