The cellular components of bronchoalveolar lavage (BAL) fluids from 26 patients with hypersensitivity pneumonitis were analyzed with regard to the period of antigen exposure before BAL procedure. In the early phase of antigen exposure BAL fluids contained increased neutrophils as well as lymphocytes while a low percentage of neutrophils and a prominent increase of lymphocytes were observed in chronic antigen exposure. Furthermore, the in-creased neutrophils decreased rapidly after cessation of antigen exposure while lymphocytes decreased gradually. In the BAL fluids of patients with summer-type hypersensitivity pneumonitis, CD4+ percentages and CD4/CD8 rations were significantly decreased, but CD8+ percentages significantly increased during antigen exposure compared with that in healthy controls. However, the actual CD4+ cell counts in BAL fluids significantly increased while CD4+ percentage decreased. Thus, CD4+ as well as CD8+ may play important roles in the pathogenesis of hypersensitivity pneumonitis. Soluble IL-2 receptor (sIL-2R) levels in BAL fluids and serum obtained from patients with hypersensitivity pneumonitis, especially serum sIL-2R during the active phase, were signifi-cantly higher than those of normal controls, indicating that the elevated sIL-2R level reflect-ed a highly activated immune response. In relation to pulmonary fuction, the lympocyte percentage showed a significant inverse correlation with % TLC and PaCO2, while neutrophil percentage showed significant correlation with % TLC and inverse correlation with FEV 1.0%. These reaults suggest that neutrophilia in BAL fluids affects bronchobronchiolar disorders during the early phase, and lympocytosis in BAL fluids regulates organic changes of the lung in hypersensitivity pneumonitis.