Based on the finding that Langerhans cells and histiocytosis X cells react with the monoclonal antibody OKT6, raised against a subset of thymocytes, we used this antibody to study the cells collected by bronchoalveolar lavage (BAL) from 131 patients, including 18 with pulmonary histiocytosis X, 43 with pulmonary sarcoidosis, 67 with miscellaneous pulmonary disorders, and 3 controls. Immunofluorescence studies demonstrated the presence of OKT6-reactive cells in all patients with pulmonary histiocytosis X (mean +/- SEM, 5.29% +/- 1.14% of all cells in BAL fluid). Immunoelectron microscopic studies revealed that the cells labeled in these patients (n = 13) contained Langerhans granules. The number of fluorescent cells in the other 113 patients was significantly smaller (mean +/- SEM, 0.20% +/- 0.04% of all cells; P less than 0.001). In the 3 control patients, in the 43 patients with sarcoidosis, and in 61 of the 67 patients with miscellaneous disorders unrelated to histiocytosis X, no cells or less than 1% of the total were labeled; however, in the 6 remaining patients in this miscellaneous group, 1.3 to 2.8% of all cells in BAL were labeled. In 3 of these 6 patients, immunoelectronmicroscopic examination showed that the cells labeled by OKT6 had the general characteristics of Langerhans cells but lacked Langerhans granules. OKT3, OKT4, and OKT8 monoclonal antibodies did not stain histiocytosis X cells in BAL fluid.