A rabbit model of acute hypersensitivity pneumonitis was produced by immunization and subsequent inhalation of pigeon serum. The pathological features, consisting of alveolitis and interstitial granulomas, were similar to those reported previously in the experimental models. Repeated inhalation challenge of pigeon serum, however, did not develop chronic alveolitis but led to the fading of the disease. It was suggested that desensitization defined as the suppression of delayed hypersensitivity contributed to the mechanism of the fading of the disease. Repeated pigeon serum inhalation without previous immunization by the same antigen could produce a moderate interstitial pneumonitis but did not demonstrate a typical granulomatous histological change in lungs. It was necessary to use adjuvant for producing a complete animal model of hypersensitivity pneumonitis. Antigen-specific antibody response did not directly correlate with the development of the animal model of hypersensitivity pneumonitis.