A chronic pulmonary infection model in rats was employed to assess the role of individual Pseudomonas aeruginosa exoproducts in disease due to this organism. Groups of rats were inoculated transtracheally with agar beads in which were embedded approximately 10(4) colony-forming units of P. aerugijnosa PAO and the PAO derivatives PR1, T1, E64, and a mixture of T1 and E64 in equal numbers (10(4)). Eight animals from each group were sacrificed at 3, 9, and 30 days after challenge, and their lungs were examined for histopathological changes, bacterial numbers, and the presence of P. aeruginosa exoproducts. The Tox- mutant T1 and the PR1 mutant, which produces enzymatically inactive toxin A, were both found to be less virulent in the rat lung model than was the toxigenic parental strain PAO. Pathological changes seen in animals infected with these mutants were restricted to intra- and peribronchial inflammation, whereas the toxigenic parental strain caused parenchymal changes, including a dense mononuclear-cell infiltration in the alveolar spaces in addition to intra- and peribronchial inflammation. Additionally, mutant E64, which produces a temperature-sensitive elastase, was also found to be less virulent in the rat lung model than was the parental strain. These data demonstrate that both active toxin A and elastase are required for maximum virulence of P. aeruginosa in this model.