Morphologic and immunohistochemical studies were made of open lung biopsies from 9 patients with pulmonary histiocytosis X (HX) and 12 patients with other conditions, and of skin biopsies from patients with cutaneous sarcoidosis, Chester-Erdheim disease, and eruptive histiocytoma. The monoclonal antibody OKT6 was detected with the use of goat anti-mouse IgG labeled with fluorescein (FITC) for light microscopy, and sheep antimouse Fab'2 fragment of IgG labeled with horseradish peroxidase (HRP) for immunoelectron microscopy. The presence of S-100 protein was revealed by an antibody prepared against bovine S-100 protein, using sheep anti-rabbit IgG labeled with FITC for light microscopy and with HRP for immunoelectron microscopy. OKT6 antibody and S-100 protein were detected simultaneously by double labeling with FITC and rhodamine. In all patients with pulmonary HX, the major cellular components (HX cells) of the granulomas showed labeling of the plasma membranes by OKT6 and of the cytoplasm by the anti S-100 protein antibody. The double-labeling technique demonstrated that the same cells carried both reactivities. Immunoelectron microscopy showed that the reactive cells had all the structural characteristics of Langerhans cells, including Langerhans cell granules. Cells reacting with OKT6 showed discrete internal labeling in some of the Langerhans granules, especially those in continuity with the plasma membranes. However, internal labeling of Langerhans granules was not demonstrated in preparations for the localization of S-100 protein. Control samples of sarcoid lesions and other pulmonary lesions unrelated to HX did not show any reactivity except in Langerhans cells; a skin lesion from a patient with eruptive histiocytoma contained OKT6-positive cells which did not have Langerhans granules.