Autoimmune hemolytic anemia (AIHA) represents the most elegant model of autoimmunity in humans. It is frequently associated with other diseases of altered immune states. By definition, antibody must be demonstrable in every case. Two types of antibody, cold and warm, are recognized. They vary considerably in chemical structure, serologic expression, extent of damage to erythrocytes, and incidence. Warm antibodies are almost exclusively gamma G globulins, and affected erythrocytes are seqestered by the spleen. Cold antibodies are macroglobulins, and affected erythrocytes are usually destroyed in the entire reticuloendothelial system. Present therapy is based chiefly on the nature of the antibody.