Factor IX enhances thrombin generation in factor XI–depleted plasma. (A) Factor XI–depleted plasma was supplemented with 1 U/ml prothrombin (90 µg/mL), factor X (8 µg/mL), factor V (10 µg/mL), factor VIII, or factor IX (Benefix; 5 µg/ml). Thrombin generation was initiated with 1 pM tissue factor. (B) Factor XI–depleted plasma was supplemented with 2.5 or 5 µg/mL factor IX (FIX B, Benefix) in the presence or absence of a blocking anti-factor IX antibody (Moab aFIX). Thrombin generation was initiated with 1 pM tissue factor. (C) Factor XI–depleted plasma was supplemented with factor IX (FIX B, Benefix) at the indicated concentrations. Thrombin generation was initiated with 1 pM tissue factor. (D) Factor XI–depleted plasma was supplemented with factor IX (FIX N, Nonafact) at the indicated concentrations. Thrombin generation was initiated with 1 pM tissue factor. F, factor.