Murine peritoneal macrophages were isolated and their ability to restrict growth of a virulent Mycobacterium tuberculosis in response to IFN-gamma was assessed in various conditions. Doses of IFN-gamma ranging from 10 to 100 U stimulated high levels of antimycobacterial activity, as seen by inhibition of growth. Addition of catalase, superoxide dismutase, and other scavengers of reactive oxygen species before infection failed to abrogate this restriction of growth, suggestive of a lack of involvement of reactive oxygen species in this phenomenon. Addition of arginase before infection inhibited the bacteriostatic ability of IFN-gamma-pulsed macrophages as did addition of NG-monomethyl L-arginine, an inhibitor of the synthesis of inorganic nitrogen oxide. In both cases, this inhibition was reversed by adding excess L-arginine in the medium. Moreover, nitrite production in macrophages was correlated with their ability to restrict tubercle bacilli growth. These results imply that nitric oxide or another inorganic nitrogen oxide is an important effector molecule in restricting growth of M. tuberculosis in IFN-gamma-pulsed murine macrophages.