Abnormalities in the proportions of immunoregulatory T-lymphocytes in bronchoalveolar lavage fluid have been reported in interstitial pulmonary diseases, yet the effect of cigarette smoking on T-cell subsets in bronchoalveolar lavage fluid from normal subjects has not been investigated. We applied an immunoperoxidase technique performed on glass slides using the monoclonal antibodies, OKT3, OKT4, OKT8, OKIa, and Leu-7 to study T-cell subsets in bronchoalveolar lavage fluid and blood of 11 normal nonsmokers and 12 smokers. In the bronchoalveolar lavage fluid of smokers, a decrease in the percentage of OKT4+ helper/inducer and an increase in OKT8+ suppressor/cytotoxic cells resulted in a markedly decreased OKT4/OKT8 ratio (0.9 +/- 0.4) compared with nonsmokers (1.9 +/- 0.8) (p less than 0.005). These abnormalities of lymphocyte subsets in bronchoalveolar lavage fluid correlated with the cumulative pack-years of smoking history but were not reflected in the peripheral blood. These results suggest that cellular immunoregulation is disturbed in the lungs of cigarette smokers. This may play a role in pulmonary defense mechanisms and tumor immunity.