Factor VIII coagulant moiety binds to platelets by binding to phospholipids of the platelet membrane. 
Washed human platelets were incubated with commercial factor VIII concentrate, or with purified factor VIII coagulant moiety.  Platelets were then washed again and lysed by sonication.  VIII:Ag and vWf:Ag were measured in the platelet lysate prior to and after incubation of the lysate with phospholipase C (PL-C).  Platelet bound VIII:Ag was significantly higher after incubation of washed platelets with factor VIII concentrate than after incubation with buffer.  Platelet bound VIII:Ag was further increased when platelets had been incubated with concentrate in the presence of thrombin and collagen.  In contrast, only a slight increase in platelet bound vWf:Ag was observed after incubation of platelets with concentrate.  When washed platelets had been incubated with factor VIII coagulant moiety, also significantly more platelet bound VIII:Ag was observed than after incubation with buffer.  Measurable VIII:Ag, but not vWf:Ag, increased significantly after incubation of the platelet lysate with PL-C. When intact washed platelets had been treated with PL-C prior to the incubation with concentrate, binding of VIII:Ag to platelets was nearly completely abolished.  Our data suggest that the factor VIII coagulant moiety binds to phospholipids of the platelet membrane and thereby contributes to the assembly of the factor X activating complex on the platelet surface.