Lipopolysaccharide (LPS) is responsible for the tissue injury that occurs following the invasion of multicelluar organisms by Gram-negative microbes. The effect of LPS on IFN-$gamma$-induced chemokine Mig gene expression in mouse peritoneal macrophages was investigated. Very little Mig mRNA was detectable upon exposure to LPS without IFN-$gamma$. Although LPS alone is only minimally effective, LPS plus IFN-$gamma$ synergized to produce a high level of Mig mRNA in the peritoneal macrophages. This synergy was not dependent on a new protein synthesis, and was not controlled at the level of the gene transcription. Futhermore, LPS did not increase IFN-$gamma$-induced Mig mRNA stability. Accordingly, it is suggested the LPS may synergize the expression of IFN-$gamma$-induced Mig mRNA through a process that depends on a pretranscriptional level or concurrent Mig mRNA translation.