After exposure of the temperature sensitive ts-1 mutant of respiratory syncytial virus to the chemical mutagne, 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 degrees C. Subsequently it was shown that the 2 clones grew less well at the restrictive temperatures of 37 degrees and 38 degrees C than did the ts-1 parent. Peak titers of the NG derived clones were decreased 10--30 fold at 37 degrees C and over 100-fold at 38 degrees C compared to ts-1. Complementation analysis indicated that the NG mutants retained the same complementation pattern as the ts-1 parent.