Pathogenic mycobacteria survive within macrophages despite T cell responses that activate host defenses against most pathogens. Among cytokines produced by T cells, IL-10 is known to negatively regulate Th1 cells as well as macrophages. IL-10 has been shown to inhibit the anti-mycobacterial activity of macrophages in vitro and could account for the ability of mycobacteria to survive intracellularly. To test the inhibitory functions of IL-10 in vivo, transgenic mice that secrete IL-10 from the T cell compartment were constructed and infected with Calmette-Guérin bacillus (Mycobacterium bovis). These mice were unable to clear the infection and developed large bacterial burdens. Nonetheless, their T cells produced abundant amounts of IFN-gamma and IL-2 in response to Ag challenge. These results indicate that the presence of excess IL-10 had little, if any, effect on T cell function or development during the immune response to Calmette-Guérin bacillus. Rather, the data suggest that IL-10 helps maintain mycobacterial infections by acting primarily at the level of the macrophage, overriding anti-mycobacterial signals delivered by IFN-gamma.