Acatalasemic mice are no more sensitive, as measured by lethality, than normocatalasemic mice to acute whole-body x-irradiation, chronic whole-body gamma irradiation, or exposure to one or several atmospheres of 100% oxygen. If the animals receive a preliminary injection of aminotriazole, which reduces the catalase activity of all solid tissues but does not affect erythrocyte catalase, the acatalasemic mice are rendered essentially acatalatic, but are still no more sensitive to the agents and procedures listed. In fact, the data suggest that low body catalase, far from being deleterious, may actually be protective to an animal receiving a single dose of whole-body x-irradiation. On the other hand, the acatalasemic mouse is considerably more sensitive to injected hydrogen peroxide than is the normal mouse, and prior injection of aminotriazole further increases this sensitivity.