Fibronectin is a large dimeric glycoprotein found in plasma, on cell surfaces and as a component of the extracellular matrix which has been implicated in a variety of adhesive processes. The role of fibronectin in platelet function has not been clarified. The present investigation demonstrates that an excess of exogenously added fibronectin inhibits platelet aggregation induced by either thrombin or A23187 at a step subsequent to platelet activation and secretion. Similar concentrations of fibrinogen or von Willebrand factor, both of which also bind to the surface of activated platelets, are not inhibitory. These results are consistent with the concept that fibronectin is one of the important mediators of platelet aggregation.