Dose dependent pulmonary lesions of acute bronchopneumonia were induced in male, outbred Swiss Webster mice by intrabronchial inoculation of Pasteurella haemolytica. Five exponential dilutions ranging from 5 x 10(4) to 5 x 10(8) colony forming units per mL (CFU/mL) of Pasteurella haemolytica serotype 1 were inoculated into five groups of mice. Mice were killed by cervical dislocation 24 hours postinoculation. Pulmonary lesions occurred in mice of all five groups, however, 5 x 10(7) CFU/mL was the minimal dose which consistently produced lesions. Focal parenchymal necrosis, suppurative bronchiolitis, and flooding of interalveolar septa and alveoli by edema fluid, fibrin, neutrophils and macrophages, were observed microscopically. We conclude that outbred Swiss Webster mice can be used as a model for the study of selected disease mechanisms of acute lung inflammation and that this model may be used to determine some of the pathogenic mechanisms involved in the development of pulmonary lesions in bovine pneumonic pasteurellosis.