Host defense mechanisms against Mycobacterium avium complex (MAC) are poorly understood. Recent evidence suggests the role of NK cells in the host defense against some intracellular pathogens. We investigated whether NK cells play a role in MAC infection. IL-2-activated human NK cells were incubated with human monocyte-derived macrophages either before or after infection with MAC. Macrophages were lysed 3 and 5 days after infection for quantitation of viable intracellular organisms. Although no killing was observed by nonstimulated macrophages, exposure to IL-2-treated NK cells for 24 h before infection induced macrophage to kill 70 +/- 8% of intracellular MAC by 3 days, and 81% +/- 4% in 5 days (p less than 0.01 for both compared with control). Killing was not blocked by incubation with anti-TNF antibody (Ab) or anti-IFN-gamma Ab. Similarly, incubation of macrophages for 24 h with supernatant obtained from IL-2 activated NK cells was associated with 74 +/- 4% killing of intracellular MAC in 3 days and 81 +/- 6% in 5 days (p less than 0.01 for both compared with control). However, the supernatant-mediated activation was partially blocked by anti-TNF Ab (46 +/- 6%; p less than 0.05) but not by anti-IFN gamma Ab. When infected macrophages were incubated with NK cells 24 h after infection for 48 h, they killed 54 +/- 3% of intracellular M. avium in 3 days and 73 +/- 5% in 5 days (p less than 0.02 for both compared with control). This effect was also not blocked by either anti-TNF or anti-IFN gamma Ab. These results suggest that activated NK cells may have an important role in the intracellular killing of MAC and that the NK-mediated activation of macrophages is in part mediated by TNF.