A modified erythrocyte insulin receptor assay was developed. The major advantage of our method is the smaller amount of blood necessary for the assay and consequent simple handling in the binding assay. Three ml of blood is enough for this assay, making it possible to assess insulin receptors in a group of pediatric patients. There was a close relationship between mononuclear cells and erythrocyte insulin binding assays (r = 0.900, p < 0.05). With this method insulin binding studies were done on normal subjects and adult onset non-obese diabetics. Decreased insulin binding was seen in the diabetic group, and this was due to a decreased number of insulin receptors. This erythrocyte insulin receptor assay can be useful tool for the study of the mechanism of insulin resistance.