Plasma and therapeutic preparations of factor VIII (1 recombinant factor VIII and two monoclonally purified plasma-derived factor VIII preparations, Kogenate, and AHF-M and Monoclate, respectively) were centrifuged in a sucrose density gradient, and the fractions were analyzed for factor VIII and von Willebrand factor (vWF). The residual vWF in the monoclonally purified factor VIII preparations sediments more slowly than the vWF of plasma. In the absence of added vWF, the factor VIII in all preparations sediments more slowly than plasma factor VIII. These same preparations of factor VIII added to hemophilic plasma as a source of vWF sediment differently. The addition of either recombinant factor VIII or AHF-M results in sedimentation of the factor VIII with the plasma vFW and in a position indistinguishable from factor VIII in plasma. In contrast, when Monoclate is added to hemophilic plasma in vitro, the factor VIII sediments more slowly than the vWF of the hemophilic plasma. However, 5 min after the infusion of Monoclate into a patient with hemophilia A, the factor VIII sediments with the plasma vWF.These results indicate that the addition of recombinant factor VIII and AHF-M results in random binding to all vWF multimers of plasma, while there is little exchange between the added factor VIII in Monoclate and the plasma vWF in vitro. In contrast, when the Monoclate is infused, there is rapid binding of factor VIII to the plasma vWF. This is presumably due to tight binding of the factor VIII to the vWF in Monoclate, whereas the factor VIII in Kogenate and AHF-M, containing no or very low amounts of vWF, are free to bind to plasma vWF. In vivo there are other conditions, as yet uninvestigated, which allow for the binding of factor VIII to vWF.