Sialidases catalyse the hydrolysis of terminal sialic acid of the carbohydrate moiety of glycoconjugates. Sialic acids play a key role in the expression or masking of antigenic sites and in cell-cell interactions. As an example, removal of sialic acid from the human erythrocyte membrane unmasks underlying molecules such as the specific carbohydrates (Gal-GalNac) of the so-called T or Thomsen-Friedenreich cryptic antigen. A consequence of this, is the recognition of that antigen by natural serum antibodies. Since the T antigen has been shown to be present in the lung, we have investigated the possible presence of sialidase and of specific antibodies to sialidase-treated cells in bronchoalveolar lavage fluids (BALF) from patients with pulmonary sarcoidosis or idiopathic pulmonary fibrosis (IPF). By using a fluorogenic substrate (4-methyl umbelliferyl-alpha-D-N-acetyl sodium neuraminate), we were able to detect a sialidase activity in BALF from eight out of nine patients with IPF and from ten out of thirty-five patients with sarcoidosis. BALF from normal volunteers and serum from both patients and normal volunteers were devoid of activity. BALF sialidase has an optimum pH activity of 5.4, it is not inhibited by EDTA and has a molecular weight close to 21 kD. BALF anti-T antibodies (galactose specific) were detectable in minute amounts in only one out of the nine normal volunteers. By contrast, they were frequently present in BALF from sarcoidosis (77%) or IPF (66%) patients and sarcoidosis patients had a higher mean activity. No correlation was observed between the enzymatic and antibody activities.