A monoclonal antibody (MAb) to von Willebrand factor antigen (vWF : Ag) was produced. In the present study, we used the MAb for an enzyme-linked immunosorbent assay (ELISA) to quantify vWF : Ag. The amount of vWF : Ag bound to a rabbit rabbit antibody coated on a plate was measured by applying sequentially MAb, a peroxidase-labeled antibody to murine immunoglobulin and enzyme substrate. The assay is quick and simple.Cryoprecipitates were prepared with three thawing (slow-thaw, rapid-thaw and modified thaw-siphon) methods. ELISA for vWF : Ag and an assay for factor VIII clotting activity (VIII : C) were performed on cryoprecipitates and on cryosupernatants. The recoveries of vWF : Ag and VIII : C in the cryoprecipitate by the modified thaw-siphon method were the highest among the three methods.The ratio of VIII : C to the content of vWF : Ag was high in the cryosupernatant and low in the cryoprecipitate.