Monocyte insulin receptor binding was studied in six nonpregnant control patients and in 40 pregnant patients with varying degrees of carbohydrate tolerance. Competitive binding assays were performed to determine insulin binding to monocytes. Fasting insulin levels were determined. We obtained the following results: (1) When compared to values not associated with pregnancy, the number of insulin receptor sites per cell increases twofold (31,000 versus 16,300); (2) Class A diabetic patients have higher numbers of receptor sites than normal pregnant patients (80,800 versus 31,000; (3) untreated Class B diabetic patients have markedly reduced receptor sites (4,575) and bind less insulin at physiologic concentrations (p less than 0.01); (4) insulin therapy of previously untreated Class B diabetic patients restored the number of receptor sites to normal pregnant levels (29,700); and (5) Classes C and D diabetic patients had similar numbers of receptor sites (30,140) and showed a greater receptor affinity for insulin than pregnant control subjects (p less than 0.01).