Factor VIII (fVIII) is an essential component of the intrinsic pathway of blood coagulation. In this pathway, thrombin-activated fVIII (fVIIIa) functions as a cofactor for a serine protease factor IXa (fIXa), and their complex (Xase complex)activates factor X (fX) to factor Xa (fXa). In its turn, fXa participates in activation of prothrombin into thrombin, the key enzyme of the coagulation cascade. The role of fVIIIa is to increase the catalytic rate constant of conversion of fX into fXa by several orders of magnitude. As the vast majority of catalytic complexes of the coagulation cascade, the Xase complex is assembled and functions on a phospholipid surface. Until recently, cell membranes, particularly those of activated platelets, were considered the only source of phospholipid surface in physiological conditions.