Bronchoalveolar lavage (BAL) was carried out before and after antigen inhalation in ten hypersensitivity pneumonitis (HP) and five control subjects. Control subjects did not show any significant variation in BAL cells after challenge with diluted pigeon serum. In HP patients, the total number of BAL cells increased from (41.5 +/- 16.8) X 10(4) cells/ml before challenge to (84.0 +/- 28.9) X 10(4) cells/ml after challenge. Of greater interest, the values of polymorphonuclear neutrophils in patients increased from 8.3 +/- 9.7 percent before challenge to 41.2 +/- 24.35 percent, 24 hours after antigen challenge (p - 0.0001). Another BAL, carried out a week later, recovered a persistently high number of cells, (74.67 +/- 33.36) X 10(4) cells/ml. However, the percentage of polymorphonuclear neutrophils and lymphocytes did not differ from the initial pre-challenge BAL. Our study demonstrates an immediate and transient neutrophil alveolitis after antigen inhalation in patients with acute hypersensitivity pneumonitis.