APC-cofactor activity of protein S. ETP calculated from thrombin generation curves initiated with 14 pM tissue factor, 10 μM phospholipid vesicles (20/60/20 DOPS/DOPC/DOPE), 16 mM CaCl₂, and indicated concentrations of APC (final concentrations), in normal pooled plasma with (○) or without (•) anti–protein S antibodies. ETP are presented as percentages of the ETP determined in plasma without added APC.