Pleural tuberculosis constitutes a human model of local protective immunity to mycobacterial infection as the disease is usually self-limited and recurrent pleurisy is rare. To identify potentially protective antigens of Mycobacterium tuberculosis, 37 human pleural fluid B cell clones were established using EBV and their supernatants assayed by ELISA and Western blot for antibody reactivity with M. tuberculosis sonicate and culture filtrate. One antibody identified 29,000, 31,000, and 33,000 bands in culture filtrate, and 31,000, 33,000, and 47,000 bands in sonicate; its species reactivity by ELISA was limited to M. tuberculosis. Eight antibodies identified a 31,000 band in culture filtrate and a 68,000 band in M. tuberculosis sonicate, suggesting recognition of a secreted antigen. The species crossreactivity of these eight antibodies extended to M. avium. Six antibodies identified multiple bands and had crossreactivity that included M. avium and M. kansasii. There was no reactivity with recombinant M. tuberculosis 65,000 antigen. Tuberculous pleurisy may prove useful in the identification of potentially protective mycobacterial antigens, particularly those secreted during active infection, and thus accessible to the human immune response.