The incubation of 0.5% suspension of fresh normal erythrocytes with hemin or bilirubin resulted in substantial hemolysis. The amount of hemolysis achieved depended on the concentration of the lytic agents. In each concentration maximum hemolysis was reached within half an hour. The hemolytic effect was somewhat dependent on temperature. Comparison with the hemolytic effect of hemin on mice (Chau, A.C. and Fitch, C.D. (1980) J. Clin. Invest. 66, 856-858) showed that although both cells undergo hemolysis by hemin, the behaviour of each red cell type is different. Centrifugation and fluorescence quenching of membrane embedded probe revealed that both hemin and bilirubin bind to the red cell membrane, hemin having higher affinity. The reaction was found to be hydrophobic and therefore independent of ionic strength. The high affinity of the membrane for hemin was shown by its ability to compete successfully with globin for hemin. Electron microscopy of the red cells which underwent hemolysis indicated cell damage and some membrane destruction. Red cell ghosts were totally disrupted when saturated with hemin. These results suggest an explanation for hemolytic events occurring in cases such as elevation of serum bilirubin or abnormalities leading to hemin release by hemoglobin.