In the previous paper, the methods of purification of placental coagulation inhibitor (PCI) were reported. This paper deals with the acting point of the PCI on the coagulation process. Prothrombin time, activated partial thromboplastin time, Russell's viper venom time, and recalcification time were all prolonged by PCI. Neither thrombin time nor Echis carinatus venom time was affected by PCI. Factors XII, XI, X, IX, VIII, VII and V were not inactivated by the incubation with PCI. PCI had no activities of fibrinolysis, antifibrinolysis and protein Ca. In the intrinsic pathways, activity of phospholipid seemed to be inhibited by PCI. The tissue thromboplastin activity was completely inactivated by incubation with PCI.