Effect of APC and protein S on thrombin generation. Thrombin generation was performed in protein S–deficient plasma with 100nM inhibitory antibodies against TFPI. Up to 10nM APC had no effect on thrombin generation in the absence of protein S. All concentrations generate lines that are superimposable (A). After addition of 120nM protein S (at 0-10nM APC), an APC dose-dependent effect was observed (B). The top single line represents 0 to 10nM APC in the absence of protein S. Protein S in the presence of no or 2.5nM APC generated lines that were superimposable. Conditions used are noted adjacent to the peaks to which they refer. The anticoagulant effect of 10nM APC and 120nM protein S was inhibited by polyclonal antibodies against protein S (C) or against protein C (D). PS indicates protein S; PC, protein C. Representative experiments are shown (n = 3).