For competition studies between the mouse mAbs and human Fabs the antigen at 2 mg/ml (F1-protein or V-antigen) was used to coat 96-well plates (Immulon 2HB, Thermo Electron, Milford, MA), and the plates were incubated overnight at 4uC. After washing the plates, a blocking solution (1% bovine serum albumin with 0.05% Tween 20 in PBS) was added to the plates, and plates incubated for 1 hr at 37uC. The flag-labeled human Fabs, and biotinylated-mouse mAb were allowed to bind to the antigen simultaneously for 1 hr at 37uC. To detect the presence of the human Fab, an anti-flag-M2-peroxidase conjugate was used, and to detect the amount of biotinylated mouse mAb present, a streptavidin-conjugated HRP was added for 1 hr at 37uC, before adding a hydrogen peroxidase-3,39,5,59-tetramethylbenzidine solution. The color reaction was allowed to develop at room temperature for 15 min and read at 450 nm.