A B S T R A C T To assess possible cellular mechanisms of in vitro insulin resistance in noninsulin-dependent diabetes mellitus (NIDDM), maximum insulin-stimulated glucose transport and utilization and insulin binding were measured in adipocytes isolated from weight-matched normal glycemic subjects and patients with NIDDM. Glucose transport rate was determined by measuring the amount of [U-'4C]-D-glucose taken up by incubating adipocytes at trace concentrations of glucose (300 nM), and glucose metabolism by estimating the amount of lactate, CO2, triglyceride, and total glucose carbons retained in the cells following incubating at 5.5 mM glucose. Insulin binding was measured at 50, 100, and 200 pM [mono'25 -tyrosinyl A,J4insulin. Both maximum insulin-stimulated glucose transport and utilization in adipocytes from diabetic subjects were 40% (P < 0.01) and 32% (P < 0.05) lower, respectively, than values obtained for subjects with normal glucose tolerance. In addition, the maximum capacity of glucose transport was correlated with the maximum capacity of glucose utilization (r = 0.81, P < 0.001). Furthermore, fasting plasma glucose concentrations of diabetic subjects were negatively correlated with both maximum insulin-stimulated glucose transport (r = -0.56, P < 0.05) and glucose utilization (r = -0.67, P < 0.05). Since basal glucose transport in adipocytes from diabetic subjects was also 33% lower than in adipocytes from normal subjects, there was no change in the relative ability of insulin to stimulate glucose transport. However, there was a 64% decrease in the sensitivity of the glucose transport system to insulin (P < 0.05), unrelated to concomitant Address reprint requests to Dr. James E. Foley.