The sensitivity of hypocatalasemic C3H/C-s(b)/Gen mouse (C-s(b)), a mutant of the C3H/He mouse, for radiation tumorigenesis was studied following the irradiations of Cf-252 fission neutrons (Cf-252) and Co-60 gamma-rays (Co-60). Mice of both sexes at six weeks of age were irradiated once with Cf-252 and Co-60 at the doses of 0-200 cGy. As the control, C3H/HeN mice of both sexes (C3H) were also irradiated with Cf-252 and Co-60. The mice were observed for 13 months after irradiation. The incidence of liver tumors in male C-s(b) increased semilogarithmically from 1 to 50 cGy in the Cf-252-irradiated group. By Co-60 irradiation, the tumor incidence of male C-s(b) was highest at 6 cGy and then decreased with increasing radiation doses up to 200 cGy. In contrast, the incidence of liver tumors in male C3H steadily increased by 0-200 cGy of both Cf-252 and Co-60 irradiations. The glutathione peroxidase activity in the liver of the male was higher in C-s(b) than that in C3H although catalase activity in C-s(b) was lower than that in C3H. Significantly higher TBA value and lowered antioxidant activities were also observed in neoplastic liver foci in comparison with those of its adjacent liver in male C-s(b). Therefore, the implication of oxygen radicals in hepato-tumorigenesis was strongly suggested and high sensitivity of C-s(b) to low doses of radiation would be useful to investigate the mechanism of radiation tumorigenesis.