Endothelial cells were cultured from bovine adipose microvessels, pulmonary arteries, and aortas. The effects of insulin, IGF-I, and IGF-II (MSA) on glucose uptake, neutral amino acid (AIB) uptake, and thymidine incorporation into DNA by the endothelial cells were determined. Each hormone markedly stimulated all three processes in the microvessel endothelial cells but had no effect on the larger-vessel endothelial cells. In the microvessel cells the monoclonal antiinsulin receptor antibody, Ab 47-9, was observed to specifically inhibit insulin binding in the bovine microvessel cells without having intrinsic activity on the three biologic processes that were stimulated by insulin and the IGFs. When insulin binding was first inhibited by Ab 47-9, dose-response curves for insulin were markedly shifted to the right for glucose uptake, AIB uptake, and thymidine incorporation into DNA. Similar antibody treatment had no effect on dose-response curves of IGF stimulation of any of the three processes. These data further extend the biologic actions of insulin and the IGFs in cultured microvessel endothelial cells. They also suggest that certain functions mutually stimulated by both insulin and the IGFs, ie, glucose uptake, AIB uptake, and thymidine incorporation into DNA, are substantially mediated through homologous receptors.