The effect of varying concentrations of tissue factor on thrombin generation in plasma in the absence of APC and presence or absence of protein S. Thrombin generation was initiated in normal pooled plasma with indicated concentrations of tissue factor, 10 μM phospholipid vesicles (20/60/20 DOPS/DOPC/DOPE), and 16 mM CaCl₂ (final concentrations) in the presence (•) or absence (○)of antibodies against protein S. Inset: Inhibition of the ETP by protein S as function of the tissue factor concentration. All plasma mixtures contained antibodies against protein C. Thrombin generation was quantified with the fluorogenic substrate I-1140 as described in “Materials and methods.”