The GPIIb/IIIa complex functions as the aggregation site on the platelet membrane surface. This complex has been purified, characterized biochemically and morphologically, and reconstituted into phospholipid vesicles. Fibrinogen and fibronectin bind to reconstituted GPIIb/IIIa with many of the properties that characterize their binding to intact platelets. The GPIIb/IIIa complex appears to be a member of a widely distributed family of cell-surface glycoproteins that mediate cellular interactions. The terms cytoadhesins30 and integrins39 have been suggested for the members of this family of two-subunit molecules. The aminotermini of the alpha subunits of these molecules have been sequenced and appear to be homologous. Three beta subunits have been identified for this family of receptors, indicating that many alpha subunits have a common beta subunit. The three beta subunits have been sequenced, and there is about a 40 to 50% identity among their amino acid sequences. It thus appears that the receptors mediating cellular interactions have evolved from a common ancestral gene.