[en] Blood cells have been widely used to evaluate the status of the insulin receptor in man. The receptors exhibited competitive inhibition curves and nonlinear Scatchard plots similar to those reported for insulin target tissues, such as the hepatocyte and the adipocytes. This study demonstrated specific insulin binding by the erythrocytes (RBC_s) of infants, children and adults. The total insulin bound by the RBCs from both children and adults gave a small difference over the physiologic range of insulin concentrations. blood RBC_s of infants showed greater numbers of insulin receptors per cell and significant increase in the total amount of insulin binding than that in either children (p<0.01) or adults (p<0.05) over the physiologic range of insulin concentrations. The affinities of insulin RBC_s for of infants, children and adults were similar to each other. It is clear that, the measurement of insulin binding by RBC_s may be particularly useful in the study of infants and children with disorders of carbohydrate metabolism to elucidate the role, if any, of abnormal receptor function in their conditions