Colony-stimulating factors (CSFs) stimulate the activation and steady- state mRNA accumulation of an important regulatory enzyme for macromolecular synthesis, ornithine decarboxylase (ODC). Cloned murine CSF-dependent cell lines exhibited a rapid activation of ODC enzyme activity, detectable within ten minutes of stimulations with either interleukin-3 (IL-3), GM-CSF, or G-CSF. This early phase of enzyme activation did not require early protein or mRNA synthesis. The subsequent protracted rise in ODC activity occurring four to six hours after CSF treatment was dependent on increases in steady-state ODC mRNA accumulation and de novo protein synthesis. CSF, therefore, modulates both posttranslational activation of preexisting ODC and stabilization and accumulation of ODC mRNA. Antiproliferative signals, such as cAMP or interferon-gamma (IFN-gamma), effectively inhibited the CSF-directed increase in steady-state ODC mRNA. Cotreatment of the murine NSF 60.8 cell line with IFN-gamma and GM-CSF decreased steady-state ODC mRNA greater than 80% as compared with GM-CSF-treated cells alone. IFN treatment did not cause any appreciable destabilization of mature ODC mRNA, suggesting that its major effect may be at the level of ODC mRNA transcription or posttranscriptional processing. These data indicate that the ODC gene-protein system is an important molecular locus of the effects of myeloid proliferative and antiproliferation signals.