Factor IX does not inhibit the anticoagulant activity of APC in plasma. The anticoagulant activity of APC in plasma was determined in the presence of varying concentrations of prothrombin or factor IX as indicated. Clotting was initiated with 0.25 nmol/L X-CP in the presence of 8 μg/mL PE/PS/PC. Final plasma dilution was 25%, with the concomitant reduction in prothrombin concentration to 0.35 μmol/L. Prothrombin (open symbols) or factor IX (closed symbols) was added at the concentrations indicated in the presence (▵, ▴) or absence (○, •) of 16 nmol/L APC.