Effect of patients' IgG on factor XI binding to immobilized HK measured indirectly by binding of alkaline phosphatase goat antihuman IgG to patients' IgG complexed with factor XI. The white bars (□) show that the IgG of patients 1 through 5 bound to microtiter plates coated with HK to which purified factor XI was attached, in contrast to normal IgG or IgG of a type II homozygote with no inhibitor (no. 6), which did not bind. The black bars (▪) show that normal IgG or IgG of a type II homozygote with no inhibitor which was preincubated with purified factor XI (not expected to bind to each other) did not bind to the plates coated with HK. In patients 1 through 5 whose IgG was preincubated with purified factor XI (expected to bind to each other), the binding to HK was either totally blocked or reduced (compare black bars with white bars). Shown are mean ± SD of triplicates.