From a surface active fraction of porcine lung lavage fluid, separated by discontinuous sucrose density gradient ultracentrifugation, a protein with a nominal molecular weight (MW) of 15,000 daltons was isolated by sequential extraction with several buffers, including one containing deoxycholate. A monoclonal antibody was prepared from a hybrid cell (8B5E) obtained by fusing a myeloma cell, X63.Ag8.653, with spleen cells of BALB/c mice immunized with the protein. With immunoblotting technique, the antibody was found to be specific to the 15,000 dalton protein and did not react with another surfactant-associated protein with a nominal MW of 38,000 daltons. The antibody's IgG subclass was IgG1 and the light chain was kappa. In immunohistochemical studies using biotinylated antibody, peroxidase reaction products were localized selectively at inclusions of alveolar wall cells which were located chiefly at the alveolar corners. These results strongly suggest that this 15,000 dalton protein was localized in inclusions of alveolar wall cells and did not originate from other larger surfactant-associated proteins degraded after secretion into alveolar space.