Quantitation of anti-fVIII Ab in human plasma samples with less than 1% fVIII activity. Plasma samples with predetermined anti-fVIII antibody titers of 1, 11, 106, and 140 BU/mL had their molar concentrations of antibody determined to be 94.6 plus or minus 0.08, 214.3 plus or minus 7.1, 2209.4 plus or minus 84.9, 2417.7 plus or minus 3.8 nM, respectively. Seven plasma samples designated as inhibitor-free were also analyzed and 2 of the 7, presented as samples GK1 and GK2, were measured to contain 112.9 plus or minus 1.9 nM and 71.1 plus or minus 8.5 nM anti-fVIII Ab.