Because of the finding of hemolysins of the immune body type in 2 cases of acute (acquired) hemolytic anemia and because of their disappearance as the patients improved after splenectomy, the possibility was conceived that a hemolysin might be directly related to the development of the hemolytic process.1 Immune hemolytic serum, produced in rabbits by the injection of guinea pig erythrocytes, when injected in guinea pigs resulted in fulminating, acute and subacute hemolytic states dependent on the dose of serum injected.2 In both clinical patients and experimental animals spherocytosis and increased fragility of the erythrocytes in hypotonic solutions of sodium chloride were present and regressed as the process improved. It was concluded (a) that hemolytic anemias are due to the activity of agents which can be generically called hemolysins and (b) that spherocytosis (and increased hypotonic fragility) are the result of the activity of various types of hemolytic