Recently, increased proportions of OKT 4+ helper T-lymphocytes have been reported in bronchoalveolar lavage (BAL) fluid of patients with active sarcoidosis. In this study we were interested in T-cell subsets of hypersensitivity pneumonitis, a disease characterized by a similar increase in BAL T-lymphocytes as active sarcoidosis. We applied an immunoperoxidase method performed on glass slides using the monoclonal antibodies OKT 3, 4, and 8 to study T-cell subsets in blood and BAL of eight patients with hypersensitivity pneumonitis, 11 patients with active sarcoidosis, and ten control subjects. OKT 8+ suppressor cells were found to be the predominant cell type in the BAL of patients with hypersensitivity pneumonitis and recent antigen exposure. After avoidance of further antigen exposure, suppressor cells decreased and helper cells increased. The results suggest that T-lymphocytosis in BAL of hypersensitivity pneumonitis and pulmonary sarcoidosis is mediated by different immunologic mechanisms.