Subconfluent monolayers of Vero cells in TC6-well plates were inoculated at a MOI of 0.1 PFU/cell with rhMPV or rhMPV/ΔM2-2 diluted in optiMEM. After 1 hr adsorption at 35°C, the virus inoculum was replaced with 2 ml opti-MEM. Combined cells and supernatant were collected at 24 hr intervals for 4 or 6 days, stabilized with 1× SPG and frozen at -70°C. Virus titers were determined by plaque assay in Vero cells.