Factor XI activation by FXII proteins. FXII and rFXII-▵19 (50 nmol/L) were first activated by incubation with DS500 (1 μg/mL) as described in Materials and Methods. Thereafter, 25 μL of the mixture 1 to 20 diluted were added to 25 μL of purified human factor XI (40 to 80 nmol/L) preincubated 5 minutes at room temperature with 25 μL of HK (16 to 33 nmol/L, respectively) (A) or with 25 μL of buffer (B). After a 15-minute incubation at 37°C, the reaction was stopped by adding 25 μL of MoAb OT2 (100 nmol/L) and the rate of factor XI activation was measured by adding 50 μL of the chromogenic substrate S-2366 (1 mmol/L) and recording the increase in absorbance at 405 nm. Results, expressed as the nmol/L of factor XIa generated, represent the means ± SD of four different experiments. * P< .005 as compared with the amount of factor XI activated by FXII and rFXII.