The effect of D-Arg-Gly-Asp-Trp (dRGDW), a synthetic RGD-containing peptide, on platelet adhesion and aggregate formation on various purified adhesive proteins and the extracellular matrix of endothelial cells was investigated with anticoagulated blood recirculating through a parallel-plate perfusion chamber. Aggregate formation on the extracellular matrix of phorbol myristate acetate (PMA)-stimulated endothelial cells and on collagen type I was more strongly inhibited by dRGDW at higher shear rates than at a low shear rate. Platelet adhesion to the extracellular matrix of nonactivated and PMA-stimulated endothelial cells was inhibited by dRGDW, especially at high shear rates, probably as a consequence of the inhibition of platelet spreading. Inhibition by dRGDW of platelet adhesion to von Willebrand factor, fibronectin, and fibrinogen was almost complete, indicating that platelet adhesion to these substrates is mediated through RGD-directed receptors. Platelet adhesion to laminin was not inhibited by the peptide, whereas platelet adhesion to collagen was increased as a consequence of the inhibition of aggregate formation. Our results show that dRGDW is a strong inhibitor of platelet adhesion and aggregate formation, especially at high shear rates.