After exposure of the temperature sensitive ts-1 mutant of respiratory syncytial virus to the chemical mutagen, nitrosoguanidine (NG), 2 clones of virus were recovered which were more temperature sensitive and stable genetically than the ts-1 mutant. The initial criterion used for selection of the 2 clones was decreased ability to produce plaques at 36° C. Subsequently it was shown that the 2 clones grew less well at the restrictive temperatures of 37° and 38° C than did the ts-1 parent. Peak titers of the NG derived clones were decreased 10-30 told at 37° C and over 100-fold at 38° C compared to ts-1. Complementation analysis indicated that the NG mutants retained the same complementation pattern as the ts-1 parent.
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