Macrophages produce various cytokines in response to mycobacteria, including interleukin 10 (IL-10) and tumor necrosis factor alpha (TNF-α). IL-10 has been shown to down-regulate numerous macrophage functions, including microbicidal activity against intracellular bacteria and parasites. IL-10 also inhibits interferon-gamma (IFN-γ) production and antigen-specific proliferation of Thl cells mediating immunologic resistance to mycobacterial infection. In contrast, TNF-α activates macrophages and may augment their mycobactericidal activity. In this study, peripheral blood mononuclear cells (PBMC) or blood monocytes obtained from healthy tuberculin reactors were stimulated in vitro with heat-killed Mycobacterium tuberculosis or heat-killed M. avium-intracellulare complex (MAC) to produce IL-10 and TNF-α. We studied a total of 26 clinical isolates of M. tuberculosis and 28 isolates of MAC. MAC-stimulated PBMC and monocytes released significantly larger amounts of IL-10 than those cells stimulated with M. tuberculosis. However, there was no difference in induction of TNF-α production between MAC and M. tuberculosis. When TNF-α activity was neutralized by the addition of anti-TNF-α mAb in culture, MAC still induced more IL-10 secretion than did M. tuberculosis. These findings suggest that increased production of IL-10 by MAC-stimulated monocytes may play a role in the intractable disease caused by these organisms.