Insulin receptors on erythrocytes in 59 elderly and 21 youngsters were investigated through a radioreceptor assay. Specific insulin binding to receptors was found to be decreased in the elderly group as compared with the youngsters. The high affinity binding sites averaged 82 +/- 16/RBC in the elderly group, but 109 +/- 22/RBC in the youngsters, and the low affinity binding sites averaged 1497 +/- 371/RBC and 2177 +/- 461/RBC respectively. Affinity constants did not differ significantly between the two groups, and fasting blood glucose and insulin levels were the same statistically. Our results indicate that senility might lead to a decrease in the number of binding sites on cells. A discussion on the relationship between insulin resistance and insulin receptor deficiency in the elderly is presented.