Knock-out mice with defined major histocompatibility complex (MHC) deficiencies were infected intravenously with Mycobacterium bovis bacille Calmette Guérin (M. bovis BCG) to assess the relative impact of MHC class I- and II-dependent immune responses. Heterozygous control mice were capable of controlling growth of M. bovis BCG, although infection progressed chronically, as assessed by determination of colony-forming units. Furthermore, infected controls developed granulomatous lesions at the site of mycobacterial growth and delayed-type hypersensitivity (DTH) reactions after challenge with purified protein derivative of tuberculin. In vitro, spleen cells from heterozygous control mice produced high concentrations of interferon-gamma (IFN-gamma) after restimulation with mycobacterial antigens. In contrast, the MHC class II-deficient A beta-/- mice, which are virtually devoid of functional CD4 T cells, succumbed to M. bovis BCG infection. Furthermore, A beta-/- mice lacked DTH reactions to tuberculin and only few minute picnotic lesions were formed in livers of infected mice. Finally, spleen cells from infected A beta-/- mice failed to produce measurable IFN-gamma concentrations after restimulation in vitro with various mycobacterial antigen preparations. The capacity of beta 2-microglobulin (beta 2m)-deficient mice, which are devoid of CD8 alpha/beta T cells, to inhibit growth of M. bovis BCG was only slightly affected at low inocula, although significantly higher colony-forming units were detected in spleens. These knock-out mice developed strong DTH responses to tuberculin and their spleen cells produced high levels of IFN-gamma once reactivated by mycobacterial antigens. Furthermore, in livers of infected beta 2m-deficient mice, extravascular infiltrates developed which were more diffuse than those in infected control littermates. Remarkably, the beta 2m-deficient mice were substantially more susceptible to higher inocula of M. bovis BCG than their control littermates. Our data formally prove the essential role of MHC class II-dependent immune mechanisms in all relevant aspects of immunity to M. bovis BCG. In addition, our findings emphasize an important contribution of MHC class I-dependent immunity to effective anti-mycobacterial protection. We assume that CD4 T cells are highly effective in containing M. bovis BCG within distinct granulomatous lesions, but fail to eradicate their intracellular pathogens. It appears most likely that CD8 T cells are also required to achieve this goal.