Inhibition of FVIII activity evaluated in a modified Bethesda assay. Various dilutions of the plasma of patient LE were incubated with either normal plasma (•), plasma of patient’s brother (LM; ○), a plasma-derived FVIII concentrate (▴), or rFVIII diluted in the plasma of a severe hemophilia A patient (□). Residual FVIII activity is expressed as percent of control.