Normal and acatalasemic mice were fed 0.5% clofibrate (CPIB) for 70 days and variations of catalase activity and thermal stability of liver catalase were investigated. 1. The specific catalase activity (catalase activity/g liver weight) in the liver of mice which received CPIB compared to that of controls was calculated. The specific catalase activity was in 2.2 normal mice and 1.2 in acatalasemic mice. A significant increase of activity was recognized in normal mice but not in acatalasemic mice. 2. The specific catalase activity in the blood of mice which received CPIB to that of control was determined. The specific catalase activity was 1.3 in normal mice and 1.1 in acatalasemic mice. A significant increase was not observed in either kind of mice. 3. The heat stability of catalase in the liver of normal or acatalasemic mice which received with CPIB was higher than that of normal or acatalasemic mice which did not received CPIB. 4. Increased levels of ALP and ChE activities and decreased levels of GOT activity and TG concentration were observed in the normal or acatalasemic mice which received CPIB, compared to control mice. 5. Isozyme of ALP were determined by electrophoresis. Increased ALP in the plasma of normal and acatalasemic mice was thought to be derived from liver ALP.