Piglet tracheal organ cultures were infected with respiratory syncytial virus (RSV) and observed for 21 days. Light and immunofluorescence microscopy demonstrated destruction of the ciliated epithelial cells and the presence of viral antigens in the epithelium. Virus was shed in high titer for 12--19 days. Ciliostasis could be quantitated, and it was shown that several strains of RSV grew and damaged tracheal organ cultures in a similar fashion. A temperature-sensitive mutant of RSV, ts-1, was examined at permissive (33 C) and restrictive (37 C) temperatures. This mutant, although somewhat attenuated at 37 C, was still found to cause damage to the ciliated epithelium and to replicate at both temperatures. THIS BEHAVIOR IS SIMILAR TO THAT AFTER INOCULATION OF TS-1 INTO VOLUNTEERS. This in vitro model may prove useful in the study of RSV disease and in the evaluation of candidate live virus vaccines.