To clarify the localization of KL-6 and epithelial membrane antigen (EMA) in human lungs, immune reactions to antibodies to these factors were examined in detail at light and electron microscopic levels. Immunohistochemical investigation was performed in 17 cases of usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP), hypersensitivity pneumonitis (HP), collagen vascular disease-associated interstitial pneumonias (CVD-IP), viral pneumonia, and bronchobronchioloectasis, as well as in 10 cases of presumably normal pulmonary tissue resected as a result of spontaneous pneumothorax. Immunohistochemical study revealed similar discontinuous linear or dome-shaped positive patterns restricted to type II alveolar cells in presumably normal tissue and only some regions of interstitial pneumonia. In sharp contrast, immune reactions with each of the two antibodies yielded a continuous linear pattern surrounding damaged areas in most regions of interstitial pneumonias and some normal areas as well. Staining for EMA antibody was negative in some regenerating alveolar and bronchial cells in regenerating foci in interstitial pneumonias, although staining for KL-6 antibody was always positive in these cells. Immunoelectron microscopic studies demonstrated similar positive reactions with both antibodies on the surface of alveolar epithelial cells in three of the cases examined, with surface positive granules 100-200 nm in diameter. Thus, although staining for both KL-6 and EMA antibodies exhibited discontinuous positivity restricted to type II alveolar cells in nondamaged regions, immune reactions were continuous and linear in pattern in or around damaged areas of the lungs at both light and electron microscopic levels, probably as a consequence of cell-surface barrier function. These findings in pulmonary tissue might be evidence of defense functions.