Background. Interleukin-4 (IL-4) treatment after lipopolysaccharide (LPS) induction inhibits macrophage (Mφ) IL-12 synthesis; however, IL-4 pretreatment (PreTx) primes the Mφ for increased LPS-induced IL-12 production. In this study we study the role of c-fos in the IL-4 priming of Mφ IL-12 synthesis. Methods. With a murine in vitro peritoneal Mφ model, we studied the effect of either c-fos deficiency (wild type, WT; homozygous c-fos knockout, Homo KO) or c-fos overexpression to study the role of c-fos in IL-4 priming of LPS-induced Mφ IL-12 synthesis. Results. (1) We first show that IL-4 PreTx results in a 72% decrease in Mφ c-fos mRNA compared with vehicle PreTx. (2) With respect to IL-12 p70 protein, IL-4 PreTx in the WT group increased LPS-induced Mφ IL-12 p70 2.2-fold compared with vehicle PreTx. Compared with vehicle PreTx in the WT group, vehicle PreTx in the Homo KO group followed by LPS stimulation resulted in a 2.8-fold increase in IL-12 p70 in the Homo KO group. IL-4 PreTx did not significantly increase IL-12 p70 over vehicle PreTx in the Homo KO group. (3) We studied the effect of c-fos overexpression on LPS-induced Mφ IL-12 production when primed with IL-4. Overexpression of c-fos completely inhibited IL-4 primed LPS-induced IL-12 p70 protein synthesis. Conclusions. These data demonstrated that down-regulation of c-fos is an integral part of the IL-4 priming process for Mφ IL-12 production.