The growth kinetics of NS1-mutated viruses. Single step growth curves for the NS1F, NS1F-ELCmut and ΔNS1 viruses compared to wt RSV in (a) A549 and (b) Vero cells. The cell supernatants of quadruplicate cultures were collected daily and viral titres were determined by plaque assay. The mean titres (log10 pfu/ml) are shown with standard error. In A549 cells, NS1F-ELCmut and ΔNS1 were significiantly attenuated in comparison to both NS1F and wt RSV (*P < 0.01). In Vero cells, NS1F-ELCmut was attenuated in comparison to NS1F and wt RSV (* P < 0.01), and ΔNS1 was further attenuated in comparison with NS1F-ELCmut (** = P < 0.01).