Classification of the varieties of hemolytic anemia is based upon whether hemolysis is due to inherent defects within the erythrocyte or to extracellular factors. CONFIRMATION OF DIAGNOSIS IN CASES IN WHICH THE DISEASE IS CLINICALLY SUSPECTED IS MADE UPON THE BASIS OF THE FOLLOWING LABORATORY DATA: Hyperbilirubinemia giving an indirect van den Bergh reaction, observation of typical factors in a smear of the blood, a reactive bone marrow, increased urobilinogen in stools and urine, and absence of bilirubinuria. Differentiation of the different types depends upon determination of altered fragility of erythrocytes as indicated by tests of resistance to heat, acid, and mechanical and osmotic stimuli, and upon the identification of an antibody. The importance of detecting antibodies demonstrable in acid serum is mentioned. In a majority of cases, acquired hemolytic anemia is caused by antibodies, malignant disease, or toxic agents.