Dogs with von Willebrand's disease (VWD) were infused with a highly purified human and canine factor VIII preparation. Control infusions were performed using isotonic saline solution. Following all of the concentrate infusions, the bleeding times were shortened to within the normal range for up to 4 h. Factor VIII activity rose immediately after infusion, rapidly returned to preinfusion levels, and then increased again at 18 h. The levels of factor VIII-related antigen (F VIII-RA) as measured with anticanine factor VIII also increased after infusion and then gradually declined. There was no further increase in F VIII-RA at 18 h, when the factor VIII activity showed the secondary increase characteristic of VWD. With antihuman factor VIII, two identifiable antigens were present in the plasma of the dogs given human factor VIII. The reduced platelet retention of these animals was not significantly altered following infusion, whereas ristocetin-induced platelet aggregation was corrected in one of the dogs given human factor VIII. These studies indicate that factor VIII procoagulalant and von Willebrand factor activities reside on a single molecule or form part of a molecular complex. In addition, the factor VIII preparations appeared to contain the factor VIII synthesis-stimulating factor and/or a precursor of factor VIII procoagulant activity.