To evaluate the usefulness of anti-T6 monoclonal antibody cell analysis in the assessment of diffuse lung disease, 77 bronchoalveolar lavages (BAL) were performed on 70 subjects: 18 normal smokers, 14 normal nonsmokers, 30 patients with chronic interstitial lung diseases (15 sarcoidosis, 12 idiopathic or associated pulmonary fibrosis, 3 histiocytosis X) and 8 patients with diffuse lung neoplastic disorders. The percentage of T6-positive cells was significantly higher in normal smokers than in normal nonsmokers (p less than 0.05). Positive T6 cells were absent or less than 1% in normal subjects, in patients with interstitial lung diseases and in patients with diffuse lung cancer, except in a case of desquamative interstitial pneumonitis, who had 2% of reacting cells. In contrast, such cells were always 3% or higher in the 6 BAL performed in histiocytosis X patients (p less than 0.05).