In order to know the beat stability of catalse in non-acatalasemic and acatalasemic reticulocyte, phenylhydrazine anemia and depleted anemia were induced both in non-acataltasemic and acatalasemic mice. The hemolysates from anemic blood diluted ten times with distilled water were incubated at 44℃ for 10min., thereafter changes in catalase activity were determined. (1) Both in non-acatalsemic and acatalasemic mice, the heat stability of catalase in anemic blood containing about 50 per cent of reticulocyte is lower than that of catalase in non-anemic blood containing about 2 per cent of reticulocyte. (2) Catalase in anemic blood containing about 50 per cent of reticulocyte in acatalasemic mice is more unstable for heat than that in non-acatalasemic mice. In the case of non-anemic blood rich in erythrocytes, the similar results were obtaind. (3) From the results, it is recognized that the catalase in acatalasemic reticulocyte is more unstable for heat than that in non-acatalasemic reticulocyte, suggesting that the catalase in acatalasemic mouse blood has the structural changes due to the structural gene mutation.