A ctivated protein C (APC) acts as a potent anticoagulant enzyme by inactivating Factor V and Factor VIII. In this study, protein S was shown to increase the inactivation of purified Factor VIII by APC ninefold. The reaction rate was saturated with respect to the concentration of protein S when protein S was present in a 10-fold molar excess over APC. The heavy chain of Factor VIII was cleaved by APC and protein S did not alter the degradation pattern. Factor VIII circulates in a complex with the adhesive protein von Willebrand factor. When purified Factor VIII was recombined with von Willebrand factor, the inactivation of Factor VIII by APC proceeded at a 10-20-fold slower rate as compared with Factor VIII in the absence of von Willebrand factor. Protein S had no effect on the inactivation of the Factor VIII-von Willebrand factor complex by APC. After treatment of this complex with thrombin, however, the actions of APC and protein S towards Factor VIII were completely restored. In hemophilia A plasma, purified Factor VIII associated with endogenous von Willebrand factor, resulting in a complete protection against APC (4 nM). By mixing hemophilic plasma with plasma from a patient with severe von Willebrand's disease, we could vary the amount of von Willebrand factor. 1 U of von Willebrand factor was needed to provide protection of 1 U Factor VIII. Also in plasma from patients with the IIA-type variant of von Willebrand's disease, Factor VIII was protected. In von Willebrand's disease plasma, which was depleted of protein S, APC did not inactivate Factor VIII. These results indicate that protein S serves as a cofactor in the inactivation of Factor VIII and Factor VIIIa by APC and that von Willebrand factor can regulate the action of these two anticoagulant proteins.