The structure of the factor VIII-related protein (VIII R:Ag) appears to be heterogeneous in patients with von Willebrand's disease and its variants. The ability to analyze the pattern of distribution of multimeric forms of VIII R:Ag via a sensitive technique would be useful for clinical diagnosis of bleeding disorders and would be helpful in understanding the molecular composition and function of the VIII R:Ag. The development of an electrophoretic technique to immobilize the protein, which could then be identified via autoradiography using purified anti-VIII R:Ag, would provide a sensitive probe, even in the presence of contaminating proteins.