Inhibition of FXI activation by synthetic peptides. A total of 15 μL of serial dilutions (4.5 mmol/L to 0.2 mmol/L) of peptide in PT were incubated with 15 μL of purified factor XI (80 nmol/L) for 6 hours at 4°C. Thereafter, 15 μL of HK (33 nmol/L) were added and after 5 minutes at room temperature, factor XI activation was started by adding 25 μL of FXII (2.5 nmol/L) activated as in Materials and Methods. After a 15-minute incubation at 37°C, the reaction was stopped by adding MoAb OT2 and the rate of factor XI activation was determined by adding 50 μL of the chromogenic substrate S-2366 (1 mmol/L) and recording the increase in absorbance at 405 nm. Results are expressed as the percentage of factor XIa formed by either protein, FXII (•) or rFXII-▵19 (▴) in the absence of peptide and represent the means ± SD of three different experiments.