Expression of monokine induced by IFN-$gamma$(Mig) mRNA is well-known to strictly depend on Interferon-$gamma$(IFN-$gamma$). Lipopolysaccharide (LPS) alone Is weakly effective on Mig mRNA expression in mouse Peritoneal macrophages. This study was undertaken to investigate the synergistic effect of LPS and IFN-$eta$ on chemokine Mig gene expression in mouse peritoneal macrophages. Although IFN-$eta$ alone was minimally effective, LPS plus IFN-$eta$ synergized to produce a high level of Mig mRNh. The synergistic effect of LPS and IFN-$eta$ (LPS/IFN-$eta$) on Mig mRNA expression was strain-specific. The most effective synergistic effect of LPS/IFN-$eta$ on the mRNh expression was found in simultaneous stimulation of LPS/IFN-$eta$. This synergy was modulated at the level of the gene transcription and was not dependent on a new protein synthesis. Synergistic effect of LPS/IFN-$eta$ also required the activation of $NF-_KB$. Accordingly, these data suggest that LPS/IFN-$eta$ synergizes the expression of Mig mRNA through a process that depends on a pretranscriptional level and/or coincident Mig mRNA transcription.