FVIII binding to myosin detected only in presence of VWF. FVIII activity was measured with a chromogenic FVIII activity kit. Recombinant FVIII or plasma-derived combination VWF/FVIII concentrate was added to a plate coated with myosin. Bound FVIII was detected using a chromogenic FVIII activity kit. No activity was seen when only FVIII was added (preparation 1, full-length FVIII; preparation 2, B-domain–deleted FVIII), but FVIII activity was detected in the presence of VWF.