The standard tuberculin skin test has been known as the prototype of delayed type hypersensitivity testing which is mediated by T cells and macrophages and plays an important role in the pathogenesis of tuberculosis. Tuberculosis is indeed a chronic infectious disease, but variation in the host immune responses to tubercle bacilli results in the various clinical manifestations of the disease ranging from an immunologically hyperreactive state observed in pleural fluid lymphocytes in tuberculous pleurisy to an almost totally unresponsive state observed in those severely ill with refractory tuberculosis. In tuberculous pleurisy, T cells in pleural fluid respond remarkably in vitro to PPD tuberculin whereas T cells in peripheral blood responded poorly to PPD stimulation. Compartmentalization of PPD-reactive T cells in the pleural fluid and immunosuppression by T cells and/or macrophages in the peripheral blood were responsible for this immunological difference observed between the lymphocytes in pleural fluid and those in peripheral blood of tuberculous pleurisy. In advanced, drug-resistant tuberculosis as well as in nontuberculous mycobacterial infection, the proliferative responses of T cells in vitro to PPD stimulation were impaired. This depressed T cell response was due to depressed interleukin-2 (IL-2) production and not due to depressed IL-2 responsiveness. Therefore, the addition of exogenous IL-2, returned the depressed PPD-induced lymphocyte proliferation in vitro in these patients to the level of the response observed in lymphocytes from patients with newly-diagnosed tuberculosis. Our results suggest that recombinant IL-2 offers a novel approach to the therapy of advanced, drug-resistant tuberculosis and nontuberculous mycobacterial infection. Preliminary clinical trials of immunotherapy with recombinant IL-2 reveals the effectiveness of this therapy and encourages us to extend the trial to a larger scale. Tubercle bacilli have various biological activities. Research on tuberculosis and tubercle bacilli have contributed much to the progress of biochemistry, pathology and immunology. Mycobacterium is a fascinating organism, which now presents another big appeal to those studying immunology: Study of immunological interaction between gamma delta T cells and the highly conserved protein in mycobacteria, HSP, heat shock protein will contribute to the elucidation of the mechanism of immunological surveillance and the mechanism of autoimmune diseases. In addition, it will also contribute to the development of a new mycobacterial vaccine which will give direct, protective immunity against tuberculosis.