A simple radioreceptor assay for insulin was established with sensitivity sufficient to detect 10 μU/ml of insulin in human sera. This radioreceptor assay was carried out at 0° using rabbit erythrocyte membranes. The binding assay for the standard curve was performed in the presence of insulin-free serum, since the binding of insulin was greatly reduced by the addition of serum. Under the conditions used, 38% of the initial binding of ¹²⁵I-insulin was displaced by 250μU/ml of native insulin. The sensitivity of the radioreceptor assay was dependent on the concentrations of both membrane protein and ¹²⁵I-insulin. The ratio of IRI to insulin values obtained by the present assay was 0.93±0.10 for 19 healthy and diabetic subjects.