Inhibition of binding of purified human anti-fVIII Ab to rfVIII-coupled beads by various concentrations of fVIII. Human Ab (1 nM) was incubated with concentrations of rfVIII (○) and IST fVIII (□) in the range of 0.125 to 2.0 nM and applied to FLI for detection. Observed is the dose-dependent and specific inhibition of binding of the human antibody by fVIII protein. The apparent affinity of the antibody for rfVIII was calculated to be 0.4 plus or minus 0.1 nM and for IST fVIII 0.5 plus or minus 0.1 nM.