Inhibition of factor XI activation in plasma by synthetic peptides. A total of 10 μL of serial dilutions (16 mmol/L to 0.2 mmol/L) of peptides in PBS 0.1% (wt/vol) Tween-20 (PT) were incubated with 10 μL of purified factor XI (160 nmol/L) overnight at 4°C. Thereafter, 40 μL of factor XI-deficient plasma were added and contact activation was started by adding 20 μL of kaolin (5 mg/mL). After a 20-minute incubation at 37°C, the reaction was stopped by adding 120 μL of stop solution followed by centrifugation to discard kaolin pellet. The amount of FXIIa-C1-inhibitor (□), kallikrein-C1–inhibitor (⧫), and factor XIa-C1-inhibitor (+––+) complexes generated in EDTA-plasma was determined as described in Materials and Methods. Results are expressed as percentage of the amount of complexes generated in the absence of peptides and represent the means ± SE of two different experiments in duplicate. In the inset, the results obtained with an irrelevant peptide, tested as control at 8 mmol/L, are shown. Symbols: FXIIa-C1–inhibitor (▧), kallikrein-C1-inhibitor (░), and factor XIa-C1–inhibitor (▪) complexes.