When exposed to thrombin, the adhesion of platelets to a von Willebrand factor (vWf) substrate, relative to a control substrate, is selectively increased. Adhesion to a vWf substrate is dependent upon the concentration of vWf, the duration of the adhesion assay, the concentration of thrombin, and the presence of divalent cations. The enhanced adhesion results from an action of thrombin on the platelets; no effect on the vWf substrate is involved. Once adherent to the substrate, the platelets undergo a profound change in morphology from the spiny sphere phenotype characteristic of activated platelets to a flattened and highly spread state. The adhesion of activated platelets to solid phase vWf is not inhibited by physiological concentrations of fibrinogen.