Because of containing of numerous immunocompetent cells, tuberculous pleurisy is a good model for analysis of local cellular immunity. When lymphocytes in pleural effusion were cocultured with purified protein derivative (PPD), they reacted to PPD and produced far more interleukin 2 (IL-2) and interferon-gamma (IFN-gamma) than did peripheral blood lymphocytes. Analysis using monoclonal antibody and complement revealed that at least the OKT4+/OKT8- T-cell subset is responsible for the antigen-specific IFN-gamma production in pleural fluid T lymphocytes. Tuberculous pleural fluid itself had far higher levels of IL-2 and IFN-gamma than malignant pleural fluid. Therefore, it is indicated that activated T lymphocytes in tuberculous pleural fluid concern the production of lymphokines at the morbid site. Treatment with IFN-gamma resulted in an increased percentage of human alveolar macrophages ingesting BCG and an increased number of ingested BCG in individual alveolar macrophage in patient with pulmonary tuberculosis. The IFN-gamma treatment also showed increased killing activity of alveolar macrophages. Through these studies, IFN-gamma is an essential cytokine which activates human alveolar macrophages and induces antimycobacterial activity. In conclusion, we could elucidate from the study of tuberculous pleurisy that exudative sensitized pleural fluid T-lymphocytes play a major role in the defence of tuberculosis at the morbid site.