Immune complexes formed in the airside may be involved in the early parenchymal changes in hypersensitivity pneumonitis. The present study was undertaken to compare the responses of animals after an intratracheal injection with preformed immune complexes to those of patients with acute hypersensitivity pneumonitis, with special emphasis on sequential bronchoalveolar lavage findings and the possible role of chemotactic factors in the immune complex-induced lung injury. An increased number and percentage of polymorphonuclear cells could be detected in bronchoalveolar lavage fluids of guinea pigs within 48 hr following an intratracheal injection of preformed immune complexes. Chemotactic factor activity preceded the observed increase of polymorphonuclear cells in bronchoalveolar lavage fluids, suggesting a role for chemotactic factors in the sequestration of these cells in the lung. In addition, this study confirmed the usefulness of bronchoalveolar lavage in evaluating the pulmonary findings because the changes in bronchoalveolar lavage cell populations correlated with sequential histological findings. The sequential characteristics of the involved areas were noted to be of a peribronchial or bronchiolar infiltration with polymorphonuclear cells at early stages, then alveolar sac infiltration, followed by mild infiltration of mononuclear cells into the alveolar walls. The findings suggest a possible role for chemotactic factors in the accumulation of polymorphonuclear cells, and the sequential changes of bronchoalveolar lavage and histological findings in animals are comparable to those in patients with acute hypersensitivity pneumonitis.