Activation of contact system in plasma by FXII proteins. A volume of 20 μL of FXII proteins (40 nmol/L) in PT was added to 40 μL of FXII-deficient plasma and then incubated with 20 μL of kaolin (5 mg/mL in PT) for 20 minutes at 37°C. The reaction was stopped by addition of 120 μL of stop solution (PBS, 0.1 mg/mL SBTI, 0.05% [wt/vol] Polybrene), followed by centrifugation for 2 minutes at 10,000g to discard the 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 nmol/L and represent the means ± SD of three independent experiments each performed in duplicate (n = 6). * P < .05 as compared with the amount of factor XIa-C1–inhibitor complexes generated by FXII.