Abstract 1. 1. The differences in hemolysis rate in erythrocytes from various animals were observed. 2. 2. Prostaglandin E 1 at concentrations of more than 10 −9 M, sharply reduced hemolysis induced by the frequent shaking and Superoxide radicals generated by the xanthine oxidase system. 3. 3. The degree of hemolysis induced by the hemolytic factors and the susceptibility to prostaglandin E 1 were found to be different among animal species. 4. 4. These differences may be correlated to the content of phosphatidylcholine in these erythrocytes.