To develop a model of legionnaires' disease in a host with defective cell-mediated immunity, rats were treated with subcutaneous cortisone acetate and exposed to aerosolized Legionella pneumophila. Bacterial clearance, histopathology, cell recovery by bronchoalveolar lavage, serology, and splenocyte blastogenesis to heat-killed L. pneumophila were studied in cortisone-treated rats and normal controls. Corticosteroid administration resulted in a dosage-related defect in the clearance of L. pneumophila. Cortisone-treated animals had a diffuse, progressive pneumonitis, but the influx of neutrophils to the lung, the serum antibody response, and the sensitization of splenocytes to L. pneumophila were not impaired by corticosteroids. The marked lymphocyte depletion observed in cortisone-treated animals may have contributed to defective expression of cell-mediated immunity. This model may be useful in further studies of the pathogenesis and treatment of legionellosis in the compromised host.