To determine the TCID 50 , MDCK cells were grown in 96-well plate at a density of 1.5 × 10 4 /well. On the next day, cells were washed with PBS and 10-fold diluted viruses in 100 µl DME were added into each well and incubated at 37°C for 1 h. After the incubation, cells were washed with PBS and 100 µl of DME containing 1x antibiotics (100 U/ml penicillin and 100 mg/ml streptomycin), 0.5% BSA (Sigma-Aldrich), and 0.5 µg/ml TPCK-Trypsin (Sigma-Aldrich) was added. Cells were further incubated for 60 h, and 50 µl of the supernatant was incubated with equal volume of 0.5% of PBS-washed Turkey red blood cells (Rockland Immunochemicals) for 30 min. Four replicates were performed for each dilution, and complete agglutination was scored as HA + . Virus titers were calculated by the Reed-Muench method. For MN assay, 100 TCID 50 of virus in 50 µl DME were incubated with 50 µl of threefold-diluted antibodies (60-0.082 µg/ml) at 37°C for 1 h. Cells were washed and incubated in the media as described for the HAI assay for 60 h. The MN titer was determined to be the final concentration of mAbs that completely inhibited infection.