Glycoprotein Ib (GPIb) is an intrinsic platelet membrane protein that plays a major role in platelet adherence and mediates ristocetin-dependent platelet von Willebrand factor binding. Recent reports that the platelet membrane glycoprotein complex IIb/IIIa is expressed in several cell types prompted us to look for GPIb expression in other vascular cells. Immunoperoxidase staining of human stomach and skin histologic sections with polyclonal as well as monoclonal anti-GPIb antibody revealed the presence of GPIb in the endothelial cell and smooth muscle cell layers. Western blotting using monospecific polyclonal anti-GPIb antibodies confirmed the presence of immunoreactive GPIb in human umbilical vein endothelial and bovine aortic smooth muscle cell cultures. Fab fragments of a monoclonal anti-GPIb antibody were used to immunoprecipitate [3H]leucine labeled GPIb from metabolically labeled cells. The GPIb in these cells was functional as measured by ristocetin-dependent cell agglutination and by vWF binding. Endothelial cells as well as smooth muscle cells bound 125I-labeled vWF in a ristocetin-dependent manner, with a Kd of 7.9 nM.