Human umbilical vein endothelial cells express a heterodimeric adhesion receptor complex consisting of noncovalently associated alpha and beta subunits that under reducing conditions have molecular masses of 135 kDa and 115 kDa, respectively. This complex can be isolated in pure form from an affinity matrix consisting of an Arg-Gly-Asp-containing heptapeptide and is specifically immunoprecipitated with monoclonal antibodies (mAbs) directed against the vitronectin receptor of human melanoma cells. These data suggest that this complex is one member of a large family of cell adhesion receptors. One of the mAbs, LM609, inhibits the attachment of human endothelial cells to fibrinogen, von Willebrand factor, and vitronectin yet has no effect on the attachment of these cells to fibronectin, collagen, or laminin. In addition, mAb LM609 inhibits attachment of endothelial cells to an immobilized synthetic peptide containing the Arg-Gly-Asp sequence. This adhesion receptor appears structurally similar to the IIb/IIIa glycoprotein complex expressed on platelets yet is antigenically distinct, since mAb LM609 fails to recognize IIb/IIIa glycoproteins. This receptor organizes in clusters on endothelial cells during their attachment to von Willebrand factor, vitronectin, or the Arg-Gly-Asp-containing heptapeptide. The data presented in this report suggest that Arg-Gly-Asp recognition may play a significant role in biological events associated with vascular proliferation.