Heinz bodies are insoluble inclusions of hemoglobin precipitated within the red cell. They reflect the presence of a metabolic derangement of or abnormality in the primary structure of hemoglobin. They are present characteristically in the congenital Heinz body hemolytic anemias (CHBHA). There are over 30 different molecular variants of hemoglobin underlying CHBHA. The 3 major causes for Heinz body formation and increased hemolysis are: exposure to certain chemicals and drugs, deficiency of one of the reducing systems of blood, and presence of an unstable hemoglobin. Oxidative denaturation of the hemoglobin molecule leads to Heinz body formation with the first two situations and is probably the mechanism for the precipitation of unstable hemoglobin. molecular mechanisms have been proposed and defined to underly the actual production of the Heinz body phenomenon. Heinz bodies are usually removed by the spleen so that postsplenectomy they increase in the peripheral blood. It has also been proposed that they may be actively extruded in cases of drug-induced hemolytic anemia. Heinz bodies occur in over 50% of cells in blood stained supravitally, especially with methyl violet. Heinz bodies can be generated in vitro in red cells of unsplenectomized patients by 60 minutes or more incubation with acetyl phenylhydrazine or by incubation of the sterile blood for 24-48 hours at 37 deg C. The ease with which the bodies develop on incubation depends upon the degree of instability of the hemoglobin.