MDCK cell monolayers (1.5610 6 cells at a confluency of 100% in a 12-well chamber) were washed with PBS and infected with different dilutions of virus for 1 h at 37uC. The virus inoculums were removed by washing with PBS. Cell monolayers were then overlaid with agar overlay medium (DMEM supplemented with 1% low melting point agarose and 1 mg/ml TPCK-treated trypsin) and incubated at 37uC. Visible plaques were counted at 4 days post-infection, and the virus titers were determined. All **data** are expressed as the mean of three independent experiments.