The effect of the inhibitors on activation of factor IX. Purified factor XIa was incubated with patients' IgG prior to incubation with factor IX. Activation of factor IX was measured by its ability to activate factor X in the presence of factor VIII, thrombin, phospholipids, and calcium ions. Results express factor Xa activity measured by a chromogenic assay using S2765. N IgG designates normal IgG; no XIa indicates that factor XIa was not added; and nos. 1 through 5 represent patients' number. Mean of duplicates is displayed.