Insulin receptors on erythrocytes were studied in normal and diabetic pregnant women to clarify the mechanism of insulin resistance in pregnancy. The assays of insulin receptors were performed according to the method of Kobayashi, which is a slight modification of the method of Gambhir. 125I-insulin binding showed no significant differences between normal pregnant women during the first (n=18), the second (n=15) and the third (n=54) trimesters and nonpregnant controls (n=52). There were also no significant differences between the values before and after delivery (n=8). Reticulocyte counts significantly increased in pregnant women during the second trimester and during the later periods. There was a positive correlation between 125I-insulin binding and reticulocyte counts in late pregnancy. These findings suggest that reticulocyte counts should always be considered in estimating erythrocyte insulin receptors in pregnancy. Then, insulin binding in late pregnant women with reticulocyte counts below 10% (n=20) was studied. The value was slightly decreased as compared to that in the nonpregnant controls, but difference was not significant. 125I-insulin binding in gestational diabetes (n=6) was decreased, but that in overt diabetes (n=4) was not. Patients with overt diabetes had been receiving insulin therapy. Insulin resistance in normal pregnancy cannot be explained by the changes of insulin receptors from this study. It may be due to some post-receptor abnormalities. But decreased insulin binding might be one of the factors that manifest or deteriorate gestational diabetes.