Tuberculous pleurisy is restricted to the pleural cavity and profuse pleural fluid, which contains numerous immunocompetent cells, is easily obtained. Therefore, tuberculous pleurisy is a good model for the study of local cellular immunity. The characteristics and function of lymphocytes in both pleural exudate and peripheral blood were studied. The pleural fluid had more T-lymphocytes than the peripheral blood. To evaluate predominant T-lymphocyte function in tuberculous pleural exudate, we studied the reactions of the lymphocytes to the specific antigen. When lymphocytes in pleural effusion were cocultured with purified protein derivative (PPD), they reacted to PPD and produced far more interferon-gamma (IFN-gamma) than did peripheral blood lymphocytes. Thus, exudative-sensitized lymphocytes in morbid sites reacted to the specific antigen more strongly. These observations indicate the presence of local cellular immunity in tuberculous pleurisy at the morbid site. It would be very interesting to know which T-cell subset produces IFN-gamma. When pleural fluid T lymphocytes were treated with OKT4 monoclonal antibody and complement, a significant decrease in IFN-gamma production was observed in all patients examined, whereas no definite decrease in IFN-gamma production was found after treatment with OKT8 monoclonal antibody and complement. These results suggest that at least the OKT4+/OKT8- T-cell subset is responsible for the antigen-specific IFN-gamma production in pleural fluid T lymphocytes. With the cooperation of macrophages or monocytes, pleural fluid T lymphocytes produced a significant level of interleukin 2 (IL-2) in the presence of PPD. Tuberculosis pleural fluid macrophages promoted greater IL-2 production than blood monocytes from either tuberculosis pleural fluid or blood T lymphocytes.(ABSTRACT TRUNCATED AT 250 WORDS)