von Willebrand protein subunit is first synthesized by human endothelial cells as a larger precursor of 260-kDa that is slowly cleaved to its 220-kDa form. Both the precursor and the finished product are glycosylated. Small amounts of uncleaved precursor are secreted into the culture media. Disulfide bonded polymers typical for von Willebrand protein are also formed inside the cells. In comparison to fibronectin the biosynthetic processing of von Willebrand protein is longer as a delay in secretion is observed. The presence of a higher cellular pool of von Willebrand protein as compared to fibronectin indicates a storage compartment for von Willebrand protein in the endothelial cells.