The presence of subplasmalemmal linear densities in cells of the mononuclear phagocyte system was investigated in pulmonary biopsies from 33 patients with fibrotic lung disorders. Subplasmalemmal linear densities, consisting of a thin layer of electron-dense material immediately subjacent to the inner leaflet of the plasma membrane, were found in 30 of the 33 patients, including each of 6 patients with pulmonary sarcoidosis, 18 of 19 patients with idiopathic pulmonary fibrosis, 4 of 5 patients with collagen-vascular diseases, 1 patient with pulmonary lymphangioleiomyomatosis, and 1 patient with marked interstitial pulmonary fibrosis associated with squamous cell carcinoma of the lung. Subplasmalemmal linear densities were found in epithelioid cells, macrophages, and giant cells in granulomas in the 6 patients with sarcoidosis and in alveolar macrophages in 4 of these patients. In patients with other fibrotic lung disorders, subplasmalemmal linear densities were limited in distribution to interstitial and alveolar macrophages. In all patients with sarcoidosis some of the subplasmalemmal linear densities of adjacent mononuclear phagocytes, particularly of those in granulomas, were paired and formed specialized intercellular junctions. Such junctions also were observed in macrophages in 10 of the patients with other fibrotic lung disorders. The junctions formed by subplasmalemmal linear densities differed from other types of junctional structures. Subplasmalemmal linear densities appear to function in 1) the binding of action filalar junctions, which may contribute to the immobilization of mononuclear phagocytes in granulomas and alveolar lumens.