The metalloproteinase ADAMTS13 regulates the size of released von Willebrand factor (VWF) multimers bound to endothelial cells, but it is unknown whether it can cleave plasma VWF during thrombogenesis. To address this issue, we perfused blood over immobilized VWF and used videomicroscopy to visualize an activation-independent platelet aggregation process mediated by soluble VWF at shear rates greater than 10 000 s(-1). At normal Ca2+ concentration, platelets formed rolling as well as surface-attached clusters that grew larger during the first 5 minutes but then lost more than 70% of their mass by 10 minutes. In contrast, platelet clusters were stable in size when metal ions were chelated, anti-ADAMTS13 IgG were added, or washed blood cells were perfused with purified VWF but no plasma. In the latter case, addition of recombinant ADAMTS13 reduced platelet cluster size by more than 70%. Incubating ADAMTS13 with VWF before perfusion did not prevent the initial platelet clustering, indicating that the enzyme may act on platelet-bound VWF under shear stress. At the concentrations tested, ADAMTS13 had no effect on platelet aggregates formed upon blood perfusion over collagen fibrils. ADAMTS13, therefore, may regulate thrombus size preferentially when the cohesion between platelets depends on VWF binding induced by pathologically elevated shear stress.