Respiratory syncytial (RS) virus ts mutants derived in LID were compared genetically with ts mutants produced in Glasgow and a total of 5 complementation groups were identified. Characterization of virus shed by chimpanzees infected with the ts-1, ts-1 NG-1, ts-1 NG-16, ts-2 or ts-7 mutant indicated that none of these viruses was completely stable genetically. However, evaluation of apparent ts ion "revertants" in primates indicated that these viruses (that had regained the capacity to produce plaques efficiently at restrictive temperature) were attenuated. It is likely that modifying mutations developed rather than true reversion. Experimental RS virus infection of the cotton rat was studied in detail and many similarities to human infection were noted. The cotton rat develops histopathologic evidence of disease in the nasal turbinate, bronchial and bronchiolar epithelium. This experimental rodent model of RS infection should prove useful in elucidating pathogenesis of disease. This project was previously a part of Project # Z01 AI 00024-18 LID.