Macrophage deactivating factor (MDF) and three members of the transforming growth factor-beta family (TGF-beta 1, -beta 2, and -beta 3) blocked the ability of IFN-gamma to induce release of reactive nitrogen intermediates from mouse peritoneal macrophages. Raising the concentration of IFN-gamma did not diminish the potency of the inhibitors (50% inhibition by approximately 7 nM MDF, 2 pM TGF-beta 1, 4 pM TGF-beta 2, and 8 pM TGF-beta 3). These inhibitors partially blocked induction of nitrite release in macrophages activated with the combination of IFN-gamma plus TNF-alpha, but were incapable of inhibiting when macrophages were activated by the combination of IFN-gamma plus LPS. MDF and TGF-beta 1, -beta 2, and -beta 3 inhibited IFN-gamma-induced nitrite release only if present during the induction phase; once IFN-gamma-nitrite release had commenced, addition of the same cytokines was no longer inhibitory. Maximum inhibition of synthesis over a 48-h period required that the inhibitors be present during the first 3 h of induction. Thus, cytokines can suppress as well as induce macrophage synthesis of reactive nitrogen intermediates, products with cytotoxic, antimicrobial, and vasodilatory properties.