Patients' IgG binding to immobilized factor XI as detected by a secondary alkaline phosphatase goat antihuman IgG. “Normal” represents control IgG; 1 through 5 are IgGs of patients with an inhibitor to factor XI, and 6 is IgG of a type II homozygote without an inhibitor. Shown are mean ± standard deviation (SD) of triplicates.