Pneumocystis carinii pneumonia (PCP) was provoked in the ferret, Mustela pulorius furo, by immunosuppression with daily long-term administration of cortisone acetate, 10-20 mg/kg subcutaneously for 9 to 10 weeks, Microscopically P. carinii was observed in the lungs of all 11 treated animals: mild to moderate in five and extensive disease in six. The histopathological features of PCP in the ferret included interstitial pneumonitis, scant mononuclear cell alveolitis, with abundant cysts and trophozoites visible in a focal distribution. There were few neutrophils present. Electron microscopy showed large numbers of both cysts and trophozoites in close association with type I cells. No bacterial pathogens were isolated from the lungs of immunosuppressed animals but an unexplained eosinophilic enteritis was present in treated animals. P carinii pneumonia developed without significant body weight loss during corticosteroid administration, unlike previously described studies using corticosteroid-treated rodents. Ferrets thus appear to be a 'steroid resistant' animal, like man, and therefore a more suitable model for immunological studies of host response to PCP than rodents. This new model also has practical advantages over previously described animal models of PCP, including larger lung and airway size.