Summary. von Willebrand factor (VWF) freshly released from endothelial cells is normally cleaved by the ADAMTS-13 metalloprotease to prevent the direct release of these ultra-large (UL) and hyper-reactive multimers into plasma. The balance of ULVWF proteolysis may be regulated by the amount of ULVWF released and the processing capacity of ADAMTS-13. The former associates with the size of ULVWF storage pool, sensitivity of vascular endothelial cells to stimulation, and the type of agonists, whereas the latter associates with the activity of ADAMTS-13. These parameters may vary significantly among individuals. We have determined the variations of ADAMTS-13 activity in 68 normal individuals by a flow-based assay and a static assay using ULVWF strings and recombinant VWF A2 domain as substrates, respectively. We found that the levels of ADAMTS-13 activity required to cleave the platelet-decorated ULVWF strings under flow is significantly higher than that of static assays. Normal plasma diluted to 25% significantly reduced its ability to cleave ULVWF strings under flow, whereas 2% plasma retained 48% enzyme activity in static assay. ADAMTS-13 activity varied from 33 to 100% among individuals and the variations were greater at shorter incubations of plasma with the substrate. Furthermore, the production of ULVWF from endothelial cells also varied among individuals. These results suggest that the commonly used static assays may underestimate the ADAMTS-13 activity required to cleave newly released ULVWF. They also demonstrated that the proteolysis of ULVWF may vary significantly among individuals, potentially contributing to the individual's vulnerability to thrombosis so that measurement of ADAMTS-13 may serve as a marker for TTP and other thrombotic diseases.