Transforming growth factor-beta 1 (TGF-beta 1) is a potent immunoregulatory molecule. It modulates production of cytokines, such as TNF-alpha and IL-6, and cell response to cytokine stimulation. Mycobacterium avium is an intracellular bacterium that multiplies within macrophages. The reason why M. avium can survive within macrophages is still unknown but probably is multifactorial. Exposure of M. avium-infected macrophages to rTGF-beta 1 before TNF stimulation significantly decreases the mycobactericidal/mycobacteriostatic activity associated with treatment with TNF. Infection of human monocyte-derived macrophages in vitro with M. avium strains belonging to serovars 1, 4, and 8 induced secretion of biologically active TGF-beta. The production of TGF-beta was strain dependent and was more pronounced in macrophages infected with virulent strains. Incubation of macrophages with M. avium-derived 33-kDa and 65-kDa proteins was associated with significant release of biologically active TGF-beta. Treatment of M. avium-infected macrophages with rTNF-alpha in the presence of anti-TGF-beta 1 antibody induced significantly greater killing of M. avium than did treatment of macrophages with TNF-alpha in the absence of anti-TGF-beta 1 antibody. In addition, macrophage monolayers treated with anti-TGF-beta 1 antibody before infection with M. avium showed mycobactericidal activity after stimulation with rIFN-gamma; in contrast, no effect of IFN-gamma was seen in monolayers not treated with anti-TGF-beta 1 antibody. We conclude that TGF-beta 1 produced after M. avium infection is a potent inhibitor of macrophage activation by cytokines. This inhibition may have an important role in the regulation of the immune response against M. avium.