An avirulent and a virulent strain of Mycobacterium avium were selected on the basis of their growth patterns in human monocyte-derived macrophages. The virulent 7497 M. avium grew progressively in untreated macrophages, whereas the avirulent LR/149 M. avium was killed to a moderate extent by untreated human macrophages (50% of the original infectious inoculum killed 7 days after infection). We set out to investigate the possibility of modulating these growth patterns by cytokine treatment. Application of tumor necrosis factor (TNF) (100 U/ml) led to macrophages restricting significantly the growth of virulent M. avium 7497 (tenfold decrease at 7 days). TNF was also effective at modulating positively the interaction between avirulent LR/149 M. avium and macrophages inasmuch as TNF-treated cells killed 99% of infecting mycobacteria at 7 days. Granulocyte macrophage-colony stimulating factor (GM-CSF) (100-10,000 U/ml) treatment led to macrophages being as mycobacteriostatic for virulent 7497 M. avium as TNF-alpha-treated cells (i.e., tenfold reduction in growth). Treatment of macrophages with both GM-CSF and TNF-alpha was shown to have additive effects on bacteriostatic activity on M. avium. The mechanism of killing of avirulent M. avium by TNF-alpha was shown to be dependent on the generation of reactive nitrogen intermediates, as seen by inhibition of effector mechanisms by NG-monomethyl-arginine and arginase. Moreover, there was a correlation between NO2- generation and mycobactericidal activity of macrophages. Addition of superoxide dismutase reversed the killing of avirulent M. avium by untreated or TNF-treated macrophages. This abrogation was also apparent in chronic granulomatous disease (CGD) macrophages, which were inefficient at generating reactive oxygen intermediates. Moreover, macrophages from CGD patients killed avirulent M. avium as efficiently as cells from normal individuals. We conclude from these results that 1) GM-CSF and TNF-alpha, alone or in combination, increase effector functions of macrophages against virulent and avirulent strains of M. avium; 2) reactive nitrogen intermediates seem to be involved in this effector mechanism; and 3) superoxide dismutase protected M. avium against macrophage effector function, seemingly by protecting the bacteria against endogenous superoxide anion. The implications of these findings for host resistance to atypical mycobacteria are discussed.