Mycobacterium tuberculosis is a bacterial pathogen capable of survival and replication within human macrophages. Cytotoxic T cells are thought to be important for the eradication of infected macrophages. To test this hypothesis, pleural effusion lymphocytes from patients with tuberculous pleuritis were stimulated in vitro with PPD, and proliferation and cytotoxicity were assessed by thymidine incorporation and chromium release, respectively. The level and kinetics of generation of antigen-specific cytotoxicity were measured and compared with those in autologous peripheral blood, control peripheral blood, and nontuberculous effusions. Both proliferation and cytotoxicity in tuberculous pleural effusions were augmented and accelerated in comparison to autologous or control peripheral blood. By contrast, low levels of cytotoxicity were observed in nontuberculous effusions, without evidence of accelerated kinetics. Cell subset fractionation by panning indicated that the cytotoxicity was mediated by CD4+ cells. The accelerated kinetics of induction of PPD-specific cytotoxic T cells demonstrated here suggests reactivation of in vivo generated cytotoxic T cells. These findings provide evidence that cytotoxic T cells are induced at the site of pathology in vivo and suggest that these cells play an important role in protection in vivo against infection with tuberculosis.