The effect of two monoclonal antibodies P2 (LyP 2) or P4 (LyP 4), specific for the platelet membrane glycoprotein I1 bj1Il a complex, on binding of '251-labelled fibrinogen or '251-labelled fibronectin to thrombinstimulated platelets was studied. These monoclonal antibodies are directed against different determinants on the I1 b-IIIa complex and react only with the complex and not with the individual glycoproteins. Fibrinogen binding to thrombin-stimulated platelets was significantly inhibited by P2 but not by P4. Fibronectin binding to thrombinstimulated platelets was significantly inhibited by P4 but only poorly by P2. These results indicate the presence of specific regions on the glycoprotein I1 b-I11 a complex which act as binding sites for fibrinogen or fibronectin. glycoprotein IIb or IIIa had no effect on platelet functionsRecently a number of monoclonal antibodies have been used to investigate the role of glycoproteins IIb and IIIa in platelet function [S]. Monoclonal antibodies, directed against glycoproteins I1 b and/or Ill a, inhibit platelet aggregation and binding of '251-labelled fibrinogen either completely, partially or have no effect [9-171. The degree of inhibition caused by these monoclonal antibodies appears to be linked to the position of their determinants in relation to the fibrinogenbinding site in the Ilb-IIIa complex 19-171. Recent reports suggest that the I1 b-111 a complex contains receptor sites not only for fibrinogen but also for fibronectin, thrombospondin and von Willebrand factor (vWf) [18-211. Very little is known about the regions of the 11 b-IIIa complex that serve as binding sites for these four adhesive proteins (fibrinogen, fibronectin, thrombospondin and vWf). A tetrapeptide, Arg-Gly-Asp-Ser (RGDS), was shown to inhibit the binding of three of these proteins to stimulated platelets [22, 231. It was, therefore. suggested that fibrinogen, fibronectin and vWf interact with a common site on thrombinactivated platelets. To test this hypothesis this study has examined the binding of 1251-labelled fibrinogen or '251-labelled fibronectin to thrombin-stimulated platelets in the presence or absence ofmonoclonal antibodies P2 (LyP 2) or P4 (LyP 4). These monoclonal antibodies are directed against different [71.