The effect of vitronectin on platelet aggregation has been investigated. Vitronectin inhibited both thrombin- and ADP-induced platelet aggregation in a dose-dependent manner. A monoclonal antibody (MoAb) to vitronectin increased thrombin-induced platelet aggregation. This effect of the MoAb was not mediated via the platelet Fc-receptor, suggesting that the antibody directly counteracted the inhibitory effect of vitronectin on platelet aggregation. Like some other adhesive proteins such as fibrinogen, fibronectin, and von Willebrand factor, vitronectin contains the amino-acid sequence Arg-Gly-Asp (RGD) which enables binding to the platelet membrane glycoprotein complex IIb/IIIa (GPIIb/IIIa). The results of this study indicate that vitronectin can modulate the function of fibrinogen on platelet aggregation by interfering with the binding of fibrinogen to GPIIb/IIIa in activated platelets.