Ultrastructural changes were observed in bronchioles in acute and repair stages of respiratory syncytial virus pneumonia induced in eight young calves (calf Nos. 1-8) using a bovine strain of respiratory syncytial virus. Five of the calves were Friesians and three were Hereford x Friesians and all were male. Tissues from three mock-infected control calves (two Friesian, one Hereford x Friesian) were also examined. Calves were from 3 to 6 days old at the time of first inoculation, with the exception of calf No. 8, which was 2 weeks old. In the acute stage of the induced pneumonia, evidence of respiratory syncytial virus replication and release was demonstrable in both ciliated and non-ciliated bronchiolar epithelial cells, with the virus-releasing process most obvious at 4 and 5 days after infection. Respiratory syncytial virus infection of bronchiolar epithelium was associated with various changes, including hypertrophy, hyperplasia, and formation of syncytia. Necrosis of epithelial cell structures usually appeared to be preceded by their desquamation from bronchiolar walls. Respiratory syncytial virus infection resulted in considerable damage to the bronchiolar ciliary apparatus. Such damage was seen as early as 1 day post-infection and was still obvious at 10 days post-infection. Neutrophils were closely associated with respiratory syncytial virus infected epithelial cells and evidence of neutrophil fusion with infected epithelial cells was seen. These observations suggest that neutrophils may be involved in killing respiratory syncytial virus infected cells and that neutrophils might play an important role in early antiviral defense against respiratory syncytial virus at a time when antibody levels are low and other cellular defenses are not fully in play. Bronchiolar repair was evident from 6 days after infection and was well advanced at 10 and 13 days after infection.