Human umbilical vein endothelial cell (HUVEC)-released ADAMTS-13 (a disintegrin and metalloprotease with thrombospondin repeats) and HUVEC-secreted von Willebrand factor (VWF) strings were investigated under static conditions that allow the accumulation and analysis of ADAMTS-13. The latter was released constitutively from HUVECs and cleaved the secreted and cell-anchored VWF strings progressively during 15 minutes in Ca(2+)/Zn(2+)-containing buffer. HUVEC ADAMTS13 mRNA expression was approximately 1:100 of VWF monomeric subunit expression. In contrast to multimeric VWF stored within Weibel-Palade bodies and secreted rapidly in response to cell stimulation, ADAMTS-13 was released directly from the Golgi to the cell exterior without an organelle storage site. The constitutive release of ADAMTS-13 continued at the same slow rate regardless of the presence or absence of histamine stimulation of HUVECs. Consequently, the percentage of VWF strings cleaved by ADAMTS-13 at VWF Y(1605)-M(1606) decreased as the rate of VWF string secretion was increased by cell stimulation. Blockade of HUVEC ADAMTS-13 activity by antibodies to different ADAMTS-13 domains made it possible to detect the attachment of ADAMTS-13 all along the lengths of HUVEC-secreted VWF strings. Constitutive ADAMTS-13 released from endothelial cells may contribute to the maintenance of cell surfaces free of hyperadhesive VWF multimeric strings.