Effect of recombinant fXI on thrombin generation in fXI-deficient plasma. Thrombin generation with (A-B) recombinant (1) fXI^WT, (2) vehicle, (3) fXI-Ala⁵⁵⁷, or (4) fXI-Ala^195-197; or (D) (1) fXI^WT, (2) vehicle, or (3) fXI-Ala.^83-84 Assays contain phospholipid (A,D) or gel-filtered platelets (B). (C) Activation of 25 nM fXI^WT (□, ○) or fXI-Ala^83-84 (■, ●) by 5 nM fXIIa (□, ■) or 15 nM α-thrombin (○, ●). fXIa generation was followed by cleavage of S2366, as described in “Characterization of recombinant fXI.”