SUMMARYHuman red blood cells (RBC) have been shown to have highly specific insulin receptors. We have studied the binding characteristics of insulin to these receptors in erythrocytes from normal male and female subjects on their usual diet and physical activity. There were no significant differences in insulin binding in erythrocytes from females between the two phases of the menstrual cycle. However, the receptor concentration was higher in the 2nd half of the cycle accompanied by a reduction in affinity.Binding curves of125‐I‐insulin to RBC from males were higher than females in either phases of the menstrual cycle, primarily due to an increase in receptor concentration when compared to females in the follicular phase and mediated by an increased affinity at low receptor occupancy when compared to females in the luteal phase.We speculated that the differences in the binding characteristics of125I‐insulin to RBC insulin receptors are mediated by differences in the levels of sex hor