Sandwich ELISA systems were developed to measure human von Willebrand factor (vWF). In one system the microtitre plates were coated with goat F(ab')2 to vWF. The F(ab')2 different molecular forms in the soluble phase. VWF antigen bound by the F(ab')2 antibody was subsequently determined by using horse-radish peroxidase labeled goat Fab' to vWF. In plasma 3 X 10(-4) units of vWF per ml could be detected with this system. In a different approach the antigen bound by the F(ab')2 antibody was probed by monoclonal antibodies to multimeric as well as to the reduced and carboxymethylated (RCM) form of vWF. Using these monoclonals and RCM-as well as native plasma as antigen, the total antigen and the relative proportion of multimeric forms in the sample were estimated.