SummaryA precise, reproducible radioreceptor assay has been developed for insulin receptors on human erythrocytes. The mean specific binding in 11 normal volunteers was 7.7% ± 1.6 (S.D.) per 2.25 × 109 erythrocytes. The intra and inter assay coefficients of variation are 4%. The erythrocyte insulin receptor is specific for insulin and recognises insulin analogues in proportion to their respective biological activities. The pH and temperature dependence, the negative cooperativity and the inverse relationship between specific binding and fasting plasma Insulin levels suggest it is similar to insulin receptors elsewhere. The erythrocyte insulin receptor offers a model for the study of insulin/insulin receptor interaction in man.