CD4+CD25+ regulatory T (Treg) cells cause immune suppression by inhibiting T cell effector functions and play pivotal roles not only in self-tolerance but also in immune response to parasitic microbial pathogens. Mycobacteria are major parasitic bacterial pathogens, but the role of CD4+CD25+ Treg cells in mycobacterial infection is not yet defined. In this study we found that, at the early stage of infection, depletion of CD25+ cells reduced both bacterial load and granuloma formation in mice infected with Mycobacterium tuberculosis strains, such as M. tuberculosis Erdman or M. tuberculosis Kurono. However, at a later stage of infection, bacterial burden and histopathology were similar regardless of depletion of CD25+ cells. Severe combined immunodeficient (SCID) mice reconstituted with CD4+CD25- T cells alone or a combination of CD4+CD25+ and CD4+CD25- T cells showed similar bacterial loads and survival kinetics after infection with M. tuberculosis Erdman. Consistent with in vivo data, in vitro studies revealed that mycobacterial antigens, purified protein derivative of tuberculin (PPD), failed to induce the suppressive function of CD4+CD25+ Treg cells to CD4+CD25- effector T cells, as demonstrated by the lack of response of CD4+CD25+ T cells to PPD, in mice chronically infected with Mycobacterium bovis bacillus Calmette-Guérin and M. tuberculosis.Our data show that CD4+CD25+ Treg cells have a transient effect at the early stage of mycobacterial infection but, contrary to the expectation, have little impact on the overall course of infection. © The Japanese Society for Immunology. 2010.