Von Willebrand factor molecules are flexible linear polymers composed of repeating protomeric polypeptide subunits. In the process of primary hemostasis, von Willebrand factor promotes platelet adhesion and platelet plug formation at the site of vascular injury. This biologic activity is apparently related to the multimeric size of von Willebrand factor. We simulated von Willebrand factor binding to the subendothelial surface by adsorbing purified human von Willebrand factor onto polystyrene latex particles of two different diameters, i.e., 0.312 micron and 2.02 micron. The rate and extent of 125I-labeled von Willebrand factor binding to polystyrene was similar with both size classes of latex particles. The von Willebrand factor-coated latex beads of 2.02 micron diameter, in contrast to the smaller size, induced rapid agglutination of formalin-fixed human platelets in the absence of any other aggregating agent. Von Willebrand factor was also adsorbed from human plasma onto latex particles coated with anti-von Willebrand factor antibodies. Again, only the large beads, carrying the von Willebrand factor-antibody complex, induced agglutination of fixed platelets. Shear stress promoted the rate of von Willebrand factor adsorption to latex particles. Our results suggest that adsorption to surface exposes binding sites in human von Willebrand factor for platelets.