Using ELISA, we studied anti-PPD antibody values in 31 tuberculous and 39 carcinomatous pleural effusions. Mean ODI values of anti-PPD IgG, IgA and IgM antibodies in tuberculous pleural effusions were higher than those in carcinomatous pleural effusions (p less than 0.01 in IgG and IgA, p less than 0.05 in IgM antibodies). We also analyzed the detected antigens in PPD and BCG whole cell fraction recognized by IgG antibody in tuberculous pleural effusions. Among heteromolecular components, the antigen band from 25 to 40 Kd region was most frequently stained. This antigen was heat-stable, contained sugar components, and included no disulfide bonds.