Significance of the APC-independent anticoagulant activity of protein S in plasma. Thrombin generation was initiated with 3.5 pM tissue factor, 10 μM phospholipid vesicles (20/60/20 DOPS/DOPC/DOPE), and 16 mM CaCl₂ (final concentrations) in normal pooled plasma (•) and in plasma containing 140% prothrombin (▵), 60% protein S (○), or 140% prothrombin and 60% protein S (□). All plasma mixtures contained antibodies against protein C. Thrombin generation was quantified with the fluorogenic substrate I-1140 as described in “Materials and methods.”