We have developed a new model for the investigation of platelet interaction with injured vascular endothelium. This involves the quantitative detection of platelet binding to a partially denuded endothelial cell monolayer in vitro. Porcine arterial endothelial monolayer, cultured on collagen gel containing fibrinogen and fibronectin, was partially denuded and the binding of Cr-51-platelets was measured. A synergistic increase in platelet binding was observed in the presence of fibrinogen and fibronectin. A distinct aggregation of platelets along the edge of the denuded area of the endothelial monolayer was seen. Prostacyclin (PGI2) inhibited platelet aggregation, although adhesive platelets were still present at denuded sites.