The insulin receptor apparent affinity was markedly decreased in fat cells treated with lectins specific either for D-galactose (Ricinus communis agglutinin I, RCAI), D-mannose (concanavalin A, Con A, Lens culinaris agglutinin, LCA) or N-acetyl-D-glucosamine (wheat germ agglutinin, WGA), as indicated by a rightward shift of the binding competition curves and almost lineared Scatchard plots. Limulus polyphemus agglutinin (LPA), specific for sialic acid, was ineffective. All lectins enhanced 2-deoxy-D-glucose uptake with relative bioactivities (maximal lectin effect/maximal insulin effect) of 68-86%. Insulin and lectin stimulatory effects were antagonized by specific carbohydrates used as competitors and inhibited by cytochalasin B (70 microM). Maximal effects of insulin and lectins were not additive and were completely abolished in neuraminidase-treated fat cells. Lectins did not affect insulin degradation. These data show that sialylated glycosidic moieties containing D-galactose, D-mannose and N-acetyl-D-glucosamine units are involved in both processes of insulin 'high affinity' binding and activation of glucose transport but are not implicated in hormone degradation. They suggest that N-linked carbohydrate chains of the complex type may be essential for functional insulin receptor and post-receptor systems.