Colostrum-deprived lambs were inoculated transtracheally with respiratory syncytial virus (RSV), Pasteurella haemolytica, or RSV and P haemolytica. Multiple tissues were examined by immunofluorescence to localize viral and bacterial antigens, and lungs were examined by electron microscopy for cytopathologic changes. Using immunofluorescence, viral antigen was detected only in the respiratory tract, mainly in the bronchial and bronchiolar epithelium and in the alveolar wall. Lesser amounts of viral antigen were detected in the surface epithelium of the nasal turbinates and trachea. Bacterial antigen was not detected. Ultrastructurally, the lambs inoculated with P haemolytica or with RSV and P haemolytica had increased numbers of type II pneumocytes, necrotic epithelial cells, neutrophils, and macrophages and excessive cellular debris in multiple foci in the lungs. Bacterial were seen only infrequently; they were within phagocytic vacuoles of neutrophils and macrophages and were free within pulmonary septa. In lambs inoculated with RSV or with RSV and P haemolytica, cells in the epithelium were multinucleated. Viral buds were seen on cytoplasmic membranes of ciliated and nonciliated cells of bronchial and bronchiolar epithelium. Isolated epithelial cells were necrotic. Viral nucleoprotein was prevalent in a few alveoli, free or within vacuoles of phagocytic cells. Necrotic debris and phagocytic cells were more prominent in the alveoli of lambs inoculated with RSV and P haemolytica than in those of lambs inoculated with either agent alone.