This study describes the clinical and microbiological features, localization of antigens, and pathological aspects of experimental infection of 4-week-old lambs with a combination of respiratory syncytial virus (RSV) and Pasteurella haemolytica A-1. Twenty-two colostrum-deprived lambs were divided into 5 treatment groups. Each group was given either sterile inocula or RSV and P. haemolytica alone or in combination. When given in combination, inoculation with RSV preceded that with P. haemolytica by 3 or 5 days;In lambs inoculated with P. haemolytica or RSV alone, a mild respiratory disease developed in one of four and two of four lambs of the respective groups. In lambs of the two groups inoculated with both agents and especially in lambs given bacteria 5 days after virus, a respiratory disease was observed which was more severe than that observed in lambs given either agent alone. Respiratory syncytial virus and P. haemolytica were isolated from 8 and 12 lambs respectively, from a total of 15 lambs inoculated with one or both agents. A serologic response to RSV was detected in all lambs inoculated with virus, but no antibody to P. haemolytica was detected;Pneumonic lesions were seen in all 11 lambs given both RSV and P. haemolytica. They were extensive in six of the lambs and were more severe than those in lambs given either agent alone. The lesions were qualitatively similar to those in lambs given the bacteria alone, and were most severe in lambs inoculated with bacteria 5 days rather than 3 days following RSV inoculation. The lesions were characterized as severe, acute, diffuse, exudative pneumonia with focal necrosis, hemorrhages, and fibrinous pleuritis. Lung lesions in lambs infected with RSV alone were interstitial pneumonitis, mild bronchiolitis and focal hemorrhages;In lambs inoculated with RSV alone or with both agents, specific immunofluorescence of viral antigen was detected primarily in the bronchial and bronchiolar epithelium and in the alveolar lumen. No bacterial antigens were detected in tissues of lambs inoculated with P. haemolytica alone or in combination with RSV;Ultrastructurally, lambs infected with P. haemolytica alone or with both agents had increased numbers of type II pneumocytes, necrotic epithelial cells, neutrophils and macrophages and excessive cellular debris in pulmonary alveoli. Occasionally, bacteria were found intracellularly within phagocytic cells or extracellularly within alveolar lumens or interalveolar septa. In lambs infected with RSV alone or with both agents, there were multinucleated giant cells and virus particles budding from bronchial and bronchiolar epithelial cytoplasmic membranes. Budding occurred from both ciliated and nonciliated epithelial cells. Viral nucleoprotein was seen free in alveolar lumens and within phagocytic cells;Experimental combined infection of lambs with bovine RSV and P. haemolytic A-1 produced clinical illness and lesions similar to those of natural cases of ovine acute pneumonic pasteurellosis.