Factor VIII functions as a cofactor in the intrinsic coagulation pathway and must first be activated to function optimally in this capacity. Low concentrations of thrombin activate factor VIII, and the presence of stimulated platelets is known to enhance the activation of factor VIII complexed to von Willebrand factor. The current studies show that platelets stimulated by thrombin, collagen, or calcium ionophore will increase the activation of isolated factor VIII by thrombin. Ongoing platelet release is not necessary for the enhanced factor VIII activation, nor is platelet von Willebrand factor or platelet membrane glycoproteins Ib or IIb/IIIa. Platelet membrane phospholipids, on the other hand, are important for the enhanced activation of factor VIII by thrombin because the effect of stimulated platelets is abolished by incubation of the stimulated platelets with phospholipases. These results suggest that the enhanced activation of factor VIII by thrombin in the presence of stimulated platelets may be mediated by factor VIII binding to platelet phospholipid or to a receptor whose functional integrity is dependent on surrounding membrane phospholipid.