Background. Interleukin 12 (IL-12) p70 is a heterodimeric protein (p35, p40 subunits) that promotes T-helper T(H)1-type cytokine response. In critically ill patients, after severe trauma or sepsis, IL-12 production is markedly impaired. We tested the hypothesis that deficiency of the transcription factor c-fos will increase macrophage IL-12 production. Methods. We harvested adherent peritoneal macrophages harvested from wild- type (WT), heterozygous c-fos knockout (Hetero KO), or homozygous c-fos knockout (Homo KO) mice and investigated lipopolysaccharide (LPS)-induced IL- 12 p70 protein synthesis (by enzyme-linked immunosorbent assay), IL-12 p35 and IL-12 p40 messenger RNA accumulation (mRNA) (by reverse transcriptase- polymerase chain reaction), and the transcription rate (by nuclear runoff). Results. (1) LPS treatment compared with vehicle increases c-fos mRNA accumulation 5-fold and AP-1 DNA protein binding (electrophoretic mobility shift assay), which precedes either IL-12 p35 or IL-12 p40 mRNA accumulation. (2) LPS induces a significant increase in IL-12 p70 protein, IL-12 p40 mRNA, and the transcription rate in the Homo KO group compared with either the Hereto KO or WT groups. (3) Compared with vehicle control, we demonstrate that interferon gamma priming increases LPS-stimulated macrophage IL-12 p70 protein in the Hetero KO or WT groups to the level of the Homo KO group but has no significant effect on the Homo KO group. Conclusions. These data suggest that deficiency of the transcription factor c-fos increases LPS- induced macrophage IL-12 production, possibly by simulating the effect of interferon gamma priming.