To clarify the mechanism of the shape change of human erythrocyte induced by anionic surfactants, the binding of sodium alkyl sulfates (CnH2n+1OSO3Na, n = 8, 10, 12) to human erythrocyte was studied. In the binding experiments with surfactant ion selective electrodes, it was found that all surfactants studied bound to the erythrocyte in two steps in the concentration range where the shape change occurred. From the analysis of binding isotherms, the binding constants (K-1, K-2) and the total number of sites (n(1), n(2)) for each binding were obtained. K-1 is larger than K-2 for each surfactant; both K-1 and K-2 increase with the number of the carbon atoms of the alkyl group of the surfactants. These facts suggest that the first binding is stronger than the second binding and that the hydrophobic interaction between the alkyl chain of the surfactants and the erythrocyte is the main driving force for each binding. Meanwhile, from the optical microscopic observation, it was found that the echinocyte was induced only in the concentration range of the second binding. Furthermore, to identify the binding sites on the erythrocyte membrane for the surfactants, the differential scanning calorimetry (DSC) measurements of erythrocyte ghost were done, and it was elucidated that the first binding site was the boundary lipid region of band 3 and the second binding site was the phospholipid layer of the erythrocyte membrane. These results imply that the shape change of the erythrocyte progresses via two steps of surfactant binding, first, to the membrane protein, band 3, and second, to the lipid layer of the erythrocyte, and that the membrane proteins also play an important role in the mechanism of the shape change of the erythrocyte.