Anticoagulant activity of protein S in the absence of APC in recalcified normal pooled citrated plasma. (A) Normal pooled plasma was incubated with antibodies against protein C with (•) or without (○) antibodies against protein S. After a 15-minute incubation, thrombin generation was started with 3.5 pM tissue factor, 10 μM phospholipid vesicles 20/60/20 1,2-Dioleoyl-sn-glycero-3-phosphoserine (DOPS)/1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-Dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and 16 mM CaCl₂ (final concentrations). (B) A similar experiment in which thrombin generation was measured in plasma in which 16 mM CaCl₂ was already present during the 15-minute preincubation with antibodies.