Canine von Willebrand factor (vWf) was compared to human vWf. Antisera raised against human vWf or canine vWf cross-reacted with both heterologous proteins and reactions of partial identity were seen using crossed immunoelectrophoresis. Similar patterns of multimerization were obtained for vWf from both canine and human sources using the enzyme-linked immunoelectrotransfer blot method. However, the canine protein displayed an altered electrophoretic mobility. The molecular weight of the vWf monomer was estimated by SDS-PAGE and found to be indistinguishable from that of human vWf monomer. Canine vWf is decreased in animals with clinically evident hypothyroidism and in heterozygous "carriers" of von Willebrand's disease (vWd) that display no clinical symptoms of vWd. Results expand the concept that von Willebrand's disease in dogs may be a useful model for study of vWd in humans, and that immunochemical methods established for studies of human vWf appear appropriate for studies of canine vWf.