Prothrombin inhibits the anticoagulant activity of APC in plasma. To study the influence of prothrombin on APC and APC/PTGla anticoagulant activity in plasma, we initiated clotting at the level of factor X activation using 0.25 nmol/L X-CP. Plasma was diluted to 25%, with the concomitant reduction in prothrombin concentration (to 35 μmol/L final) by the other assay constituents. Prothrombin in TBS was then added to give the final concentrations indicated. The arrow on the x-axis indicates the approximate physiological concentration of prothrombin (1.4 μmol/L). Either PE/PS/PC vesicles (left panels) or PS/PC vesicles (right panels) were used. The PE/PS/PC concentration was adjusted to give approximately the same clotting time in the absence of added prothrombin or APC as the PS/PC vesicles. The PE/PS/PC concentrations were 8 μg/mL in (A), 100 μg/mL in (C), and 8 μg/mL in (E). The PS/PC concentrations were 30 μg/mL in (B), 100 μg/mL in (D), and 30 μg/mL in (F). The prothrombin concentration dependence of the anticoagulant response to APC (A through D) or APC/PTGla (E and F) was then determined. Because of its increased potency as an anticoagulant, the concentrations of the chimera were reduced relative to APC. Final APC or APC/PTGla concentrations are indicated on the figure (nmol/L).