In 1756 healthy individuals the mean and S.D. values of blood catalase activity were 111.3 +/- 16.5 MU/l with lower blood catalase for females (107.7 +/- 14.4 MU/l, n = 880) than for males (117.9 +/- 16.8 MU/l, n = 876) while the ratios of blood catalase activity to blood hemoglobin concentration were not different (0.841 +/- 0.107 MU/g versus 0.849 +/- 0.119 MU/g). The decrease of blood catalase with age was greater in males (b = -0.084 MU/l year) than in females (b = -0.016 MU/l year). The screening of 3300 healthy citizens for hypocatalasemia yielded six families (0.18%), and three families were identified out of 1630 clinic patients. These nine families revealed 37 hypocatalasemic patients with 57.5 +/- 11.7 MU/l mean and S.D. of blood catalase activity. Similarly to the Japanese and the Hungarian actalasemic patients, the electrophoretic mobilities of catalase in erythrocytes of hypocatalasemic patients were indistinguishable from that of healthy controls.