In vitro coagulation produces a consumption of heparin co factor II (HC II) of 8-10% both in plasma from normal individuals and patients whereas antithrombin (AT) consumption ranges 40-50%. In blood from heparin treated patients consumption is similar or greater. In blood from warfarin treated patients, consumption is decreased. Addition of heparin prior to clotting has little effect on HC II consumption, but high heparin concentration reduces AT consumption. Addition of dermatan sulfate has no effect on AT consumption, but increases HC II consumption dramatically. In consumption coagulopathy, the HC II levels are as low as AT, possibly reflecting intravascular consumption accelerated by vascular glycosaminoglycans.