Growth of rhMPV and rhMPV/ΔM2 in Vero cells. A) Vero cell monolayers were inoculated with rhMPV or rhMPV/ΔM2-2 and incubated at 35°C under 1% methylcellulose in optiMEM. At 6 days p.i., the cells were fixed in methanol and immunostained with ferret polyclonal antibody directed to hMPV followed by anti-ferret horse radish peroxidase-conjugated antibody. The immunostained plaques were treated with 3-amino-9-ethylcarbazole for visualization. B) Replicate cultures of Vero cells were inoculated with rhMPV or rhMPV/ΔM2-2 at MOI of 0.1 PFU/cell and incubated at 35°C. Supernatants and cells were harvested daily for 4 days. Titers were determined by plaque assay in Vero cells. The graph represents an average +/- SD titer of three independently performed experiments.