Studies on the identification of molecules involved in the interaction of endothelial cells with their environment have led to the discovery of membrane receptors that are similar or identical to previously characterized platelet membrane glycoproteins. Some of these surface molecules belong to a family of widely distributed cell adhesion receptors, termed integrins. One of the integrins, the vitronectin receptor, a platelet glycoprotein (GP) IIb-IIIa related molecule, has now been characterized in some detail. It is a heterodimeric molecule consisting of a subunit similar, but not identical, to GP IIb and a subunit (almost) identical to GP IlIa. Alloantigens (Zwa, Zwb, and Yukb) expressed by platelet GP IlIa are also expressed by endothelial GP IlIa. The vitronectin receptor is involved in the adhesion of endothelial cells to Arg-Gly-Asp-containing immobilized proteins such as vitronectin, fibrin-ogen, and von Willebrand factor. Endothelial cells also express a molecule indistinguishable from the platelet VLA-2 or GP Ia-IIa complex which is another member of the integrin family. This molecule functions as a platelet collagen receptor, and perhaps it also functions as a collagen receptor on endothelial cells. By mediating cell-matrix contact, the vitronectin receptor, VLA-2 and other similar receptors might be involved in the anchorage of endothelial cells to their extracellular matrix.