Hypocatalasemic mice (C b s ) were examined for their life span and compared to C3H mice with reference to oxidase enzyme activity and spontaneous tumor development. N 1 -methylnicotinamice oxidase, xanthine oxidase (XO), xanthine dehydrogenase (XDH), and XO+XDH activities in 8 week-old male C b s mice were decreased as compared to the C3H counterparts. On the other hand, benzaldehyde oxidase in both sexes and xanthine dehydrogenase activity in females were significantly elevated as compared to the C3H. The total tumor incidence, number of tumors per mouse, liver tumors, and other tumor induction were significantly greater in as compared with until 499 days group in both sexes (P<0.01). The total incidence of tumors and incidence of liver tumors were not significantly different between the two strains in both sexes. The number of tumors per animal in C3H females and incidence of sarcoma in both sexes were significantly decreased as compared with the C b s case but the numbers of liver tumor per animal and ovarian tumors were significantly greater. (J Toxicol Pathol 2004; 17: 25-30)