Several cytokines have previously been shown to prime macrophages for enhanced release of oxygen radicals in response to subsequent stimulation. We now demonstrate that the presence of the macrophage-specific colony stimulating factor-1 (CSF-1) inhibits the priming of murine macrophages by a variety of agents including tumor necrosis factor alpha, granulocyte/macrophage colony stimulating factor, interferon-gamma, and bacterial lipopolysaccharide. CSF-1 is also able to reduce the respiratory burst in the absence of priming. Our results indicate that CSF-1 is a potent negative regulator of the macrophage respiratory burst which acts to oppose the priming (enhancing) action of macrophage activating agents. We propose that CSF-1 may have a potentially important and previously unrecognized, role as a physiological regulator which restricts or terminates the activation of macrophages in order to prevent an uncontrolled inflammatory reaction.