Different species of mycobacteria differ in their capacity to induce the production of tumor necrosis factor-alpha (TNF-alpha) by human monocytes in vitro. Whereas M. tuberculosis is a potent inducer of TNF-alpha, M. leprae is much less potent. TNF-alpha production is found to be associated with the availability of H2O2 generated by activated monocytes, as superoxide enhancing H2O2 concentration increases and catalase degrading H2O2 decreases TNF-alpha production. Furthermore, M. kansasii with high intrinsic catalase induce less TNF-alpha than mycobacteria with low intrinsic catalase. In vitro infection of monocytes with M. tuberculosis leads to an impairment of the antigen-presenting capacity, as determined by a reduction of antigen-induced T cell proliferation and interferon gamma (IFN-gamma) production. Of crucial importance is this impairment is the M. tuberculosis-induced down-modulation of MHC class II antigens. The role of TNF-alpha in vivo is reflected in patients with various forms of leprosy. In skin lesions of lepromatous leprosy patients TNF-alpha, interleukin 1 beta (IL-1 beta), and INF-gamma production are found to be rare, whereas these cytokines are well expressed in skin lesions of patients with tuberculoid leprosy. After multidrug chemotherapy an increase of local cytokine production is found. Taken together, these findings suggest that components of mycobacteria may interfere with local cell-mediated immune reactions in vivo. The molecular mechanisms involved in these local responses need to be defined.