By David R. Phillips, Israel F. Charo, Leslie V. Parise, and Laurence A. Fitzgerald A NEW FAMILY of cell-surface receptors has been identified that mediates a wide variety of cell-cell and cell-substrate adhesions. The first receptor to be described in this family was the glycoprotein(GP) lib-Illa complex on platelets. t6 This complex is a Ca2-dependent heterodimer that, on activated platelets, can bind one of four different adhesive proteins (ie, fibrinogen, fibronectin, von Willebrand,) s#{231}’factor LvWFJ, and vitronectin). The binding of fibrinoen allows primarily for platelet aggregation; the fibronectin and vWF may also allow for theadhesion and on the subendotheliurq. Other mem-bers of this family of adhesion receptors include the widely distributed fibronectin receptor (FnR), the vitronectin receptor (VnR), the very late activation(VLA) antigens, and three receptors found on leukocytes(ie, Mac-I, LFA-1, and p150, 95). These receptors mediate a wide variety of cellular interactions. All of these adhesion receptors consist of non-covalently associated a-subunits(eg, GP lIb) and f.-subunits (eg, GP lIla). Protein structure and amino acid-sequence data derived from cDNA clones indicate that the various aand (3-subunits in this family are homologous proteins and probably evolved from a common heterodimer precursor. Many of these receptors bind to adhesive proteins that contain a common recognition peptide sequence, Arg-Gly-Asp (RGD). In addition, GP llb-Illa binds to a second site on fibrinogen that is important for platelet aggregation. This review summarizes recent advances pertaining to the structure and function of platelet GP lib-Illa and its relationship to the other adhesion receptors.