Using [125I]insulin at 172 pmol/1 (1 ng/ml) the binding of insulin to mononuclear leucocytes isolated from peripheral blood was studied. Our present study comprised 21 healthy subjects (22-33 years old, 90-110% of ideal weight) and a comparable group of 22 obese subjects (20-37 years old, minimum 150% of ideal weight). A significant difference in insulin binding was found between the two groups, the mean specific insulin binding fraction in normals being 1.92 +/- 0.58 (s) X 10(-2) and that for the obese 1.19 +/- 0.41 (s) X 10(-2) (P less than 0.01). No correlation was found between body weight and the number of insulin receptors in the obese subjects. However, the number of insulin receptors was negatively correlated to fat cell size (P less than 0.05). Insulin receptors in subjects were also negatively correlated to fasting plasma insulin (P less than 0.05). Insulin receptors were studied in 11 obese subjects before and after 10 days of fasting. A significant increase in the number of insulin receptors was observed with a simultaneous decrease in plasma insulin to normal values. The results indicate that obesity complicated by hyperinsulinism is associated with a decrease in the number of insulin receptors compared with the normal. This finding may in part explain the decreased insulin sensitivity of the hyperinsulinaemic obese.