The influence of macrophage (M)-CSF on the production of inflammatory mediators has been examined in murine peritoneal macrophages. Cultures of macrophages treated with up to 30,000 U/ml of human rM-CSF or with 10,000 U/ml of L929-derived M-CSF did not reveal either PGE2, IL-1, or IL-6 secretion. In contrast, LPS, which served as a positive control, stimulated production of significant levels of PGE2, IL-1, and IL-6. Furthermore, Northern blot analysis of macrophage RNA revealed a strong induction of IL-1 alpha and IL-6 mRNA by LPS but not by M-CSF. Conditioned medium from macrophage cultures treated with purified L929 or human rM-CSF in combination with LPS exhibited a significant reduction of IL-1 bioactivity as compared with an LPS challenge alone. To investigate the mechanism involved in this M-CSF-dependent reduction of IL-1 bioactivity, we measured IL-1 alpha gene expression. The addition of M-CSF to LPS-treated macrophages did not affect IL-1 alpha mRNA levels suggesting that M-CSF may regulate production of an IL-1 inhibitor. This hypothesis was shown to be valid because removal of IL-1 alpha from conditioned medium of LPS plus M-CSF-treated cells allowed the detection of a nondialyzable factor that blocked IL-1-dependent thymocyte proliferation. The inhibitor appeared to be specific because it did not inhibit IL-2 and TNF bioactivities. Furthermore, this IL-1 inhibitor, which binds to cells and not to IL-1, competed with the binding of radioactive IL-1 alpha or beta to EL-4.6.1 cells. The results demonstrate that M-CSF alone does not induce proinflammatory mediators and PGE2 as was previously published. The data also suggest that M-CSF may play a role in the down-regulation of inflammatory responses.