The RSN-2 strain of RS virus grown in BS-C-1 cells was used for the isolation and characterisation of ts mutants. RSN-2 strain growth was examined at different temperatures and the degree of cell-association of the virus calculated. The effect of DM inhibitors such as actinomycin D and mitomycin C as well as the effect of the concentration of phenylalanine, arginine and serum on RS virus was studied. Thin sections of RS virus infected cells were examined by electron microscopy. Infected cells were also examined under a scanning electron microscope. Thirty-five ts mutants of the RSN-2 strain were isolated from mutagenised virus and one ts mutant from non-mutagenised virus.