A method for evaluating the activity of the von Willebrand factor (vWF) protease is described, and a clinical application is illustrated. The procedure utilizes gel electrophoresis, Western blotting, and luminographic detection methods to evaluate the distribution of vWF multimers before and after incubation of clinical samples under conditions that favor proteolysis by this enzyme. Physiologically, the high-molecular-weight multimers of vWF are cleaved by the vWF protease under conditions of high shear stress in parts of the arterial circulation; cleavage of vWF multimers is also observed after exposure of vWF to denaturing agents in vitro and thus can serve as a laboratory test for the activity of the protease. vWF protease activity is decreased or absent in patients with thrombotic thrombocytopenic purpura due to an inhibiting autoantibody, and this leads to high levels of noncleaved vWF and to life-threatening thrombosis, thrombocytopenia and anemia. The assay evaluates the activity of the protease by assessing the cleavage of VWF multimers after patient plasmas are incubated in vitro under denaturing conditions. With the use of these electrophoresis and Western blotting techniques, patient plasmas can be rapidly assessed for the activity of the vWF protease which may aid in the treatment strategy for these patients.