[Quantitative determination of Willebrand factor activity in Willebrand's disease].

Certain new data about the nature of Willebrand factors are discussed, its activity being considered as part of the activity of the antihemolytic factor VIII. An one-stage method for its determination is described based on the absence of thrombocytic aggregation in the presence of ristocetin in the patients with Willebrand disease as well as the illustration of one case.