A study of erythrocyte fragility caused by hydrogen peroxide (H(2)O(2)) of relatively high concentration was conducted by means of measuring potassium ion (K(+)) release from erythrocytes by potassium ion electrode. Erythrocytes used in this experiment were of normal and acatalasemic mice. The following results were obtained; 1. K(+) release from red blood cells of normal mice was more than that from red blood cells of acatalasemic mice. 2. In the presence of ethylalcohol (EtOH), K(+) release from red blood cells of normal mice was suppressed, while that from red blood cells of acatalasemic mice was scarcely influenced by the existence of EtOH. It is suggested that this phenomenon is due to the fact that catalase shows peroxidatic activity and decreased oxygen generation from H(2)O(2) in the presence of EtOH. 3. Discussion was made between hemolysis caused by hydrogen peroxide on one hand and fragility of red blood cell membrane and catalase activity on the other.