Factor IXa–stimulated thrombin generation in factor IX–deficient plasma in the absence of tissue factor. Thrombin generation was initiated by 1 to 100 pM of recombinant factor IXa wild-type (A), R170A (B), or R233A (C), in the presence of 8.3 μM of PC:PS vesicles, and 40 μg/mL of CTI (plasma concentrations) in factor IX–deficient plasma. Factor IXa was added just before recalcification at a final plasma concentration indicated in the figure. The time course of thrombin generation was measured as described in “Fluorogenic method for detection of plasma thrombin generation.” Thrombin generation curves representing the mean fluorescent data from replicate determinations (n = 10) are shown. Curves are identified by representative points. Peak thrombin concentration (D) and velocity index of thrombin generation (E) for each recombinant protein are plotted for comparison. Error bars represent SE.