We evaluated the in vivo biological activity of in vitro extensively glycosylated insulin (GI) with the euglycemic-hyperinsulinemic glucose-clamp technique in postabsorptive nondiabetic subjects. Insulin-mediated glucose disposal was approximately 30% lower (P less than .03) with GI (9.2 +/- 1.2 mg.kg-1.min-1, mean +/- SE) than with the nonglycosylated hormone (12.6 +/- 0.7 mg.kg-1.min-1) at comparable plasma insulin concentrations (approximately 90 microU/ml). Binding of GI to a specific receptor on circulating cells (erythrocytes and monocytes) was normal. We conclude that in vitro extensive glycosylation of insulin reduces its biological activity in vivo, as reflected by insulin-mediated glucose disposal, probably at a postreceptor level.