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Bernard-Soulier syndrome (BSS) platelets, which lack the membrane glycoprotein complex Ib-IX, do not adhere to subendothelium. The adhesion of platelets from two patients with BSS to subendothelial microfibrils (MFs) and type I collagen was compared in an in vitro assay adapted to patients with low platelet count. With both patients, platelet adhesion to MFs was strongly defective, whereas the adhesion to collagen was normal. The involvement of GPIb in the MFs-induced platelet adhesion was confirmed by the inhibitory effect of a MoAb (AN51) to the von Willebrand (vWF) factor binding domain of GPIb. The adhesion of platelets to MFs thus requires GPIb-IX and an axis MFs-vWF- GPIb-IX seems therefore to be prevalent in the reactivity of platelets with subendothelium.