Neutrophils and monocytes express cathepsin G and can also bind to activated platelets, thus they can be localized to the site of active coagulation. Previous studies have suggested that cathepsin G inactivated coagulation factor VIII (FVIII) and was thus anticoagulant. But other studies have indicated procoagulant functions for cathepsin G in activation of coagulation factor V or activation of platelets among other possible mechanisms. Therefore, it remains unclear if cathepsin G is anticoagulant or procoagulant. We investigated the effects of human neutrophil cathepsin G on FVIII/VIIIa. Cathepsin G activates FVIII to a partially active form while having only a minor inactivating effect on thrombin-activated FVIIIa. This inactivation is mostly due to decreased stability of FVIIIa since a disulfide bond that prevents A2 subunit dissociation from FVIIIa prevents any loss of activity due to cathepsin G proteolysis. FVIII that has been cleaved by cathepsin G can still be activated by thrombin if A2 subunit dissociation is prevented. Cathepsin G cleavages of FVIII are limited to a few specific sites that are mostly located near known activating and inactivating cleavage sites. Cathepsin G cleavage sites near to thrombin cleavage sites likely contribute to the partial activation of FVIII. Therefore, it is possible that cathepsin G from neutrophils and monocytes may provide some pro-coagulant effect by activating FVIII.