The effect of the inhibitors on factor XI activation as measured by a chromogenic assay using S2366 and expressed as the percentage of factor XIa activity generated in the presence of normal IgG. (A) Activation of factor XI by thrombin in the presence of dextran sulfate. (B) Activation of factor XI by factor XIIa. (C) Activation of factor XI by factor XIIa in the presence of dextran sulfate. (D) Activation of factor XI by factor XIIa in the presence of HK and dextran sulfate. “Normal” represents control IgG; 1 through 5, IgGs of patients with an inhibitor to factor XI; and 6, IgG of a type II homozygote without an inhibitor. Shown are mean ± SD of triplicates.