IFN-gamma induces cell cycle arrest and p53-independent apoptosis in primary cultured hepatocytes. However, it is not yet understood what molecules regulate the mechanism. We report here that interferon regulatory factor 1 (IRF-1) is an essential molecule in these phenomena. Hepatocytes from IRF-1-deficient mice were completely resistant to IFN-gamma in apoptosis indicated by three different hallmarks such as LDH release, DNA fragmentation and the activation of caspase-3 family. Caspase-1 expression was little detected in hepatocytes, and constitutive and IFN-gamma-induced mRNA expression of Fas or caspase-3 did not change in between wild type and IRF-1-deficient hepatocytes. Expression of IFN-gamma-inducible caspase, caspase-11, did not change either. Thus, it is unlikely that these molecules directly regulate the mechanisms. Interestingly, IRF-1-deficient hepatocytes were also resistant to IFN-gamma-induced cell cycle arrest despite IFN-gamma-induced cell cycle arrest and apoptosis are regulated by independent pathways. Results by Northern blot analysis showed that IFN-gamma-induced but not constitutive p53 mRNA expression was regulated by IRF-1. In fact, IFN-gamma did not induce cell cycle arrest in p53-deficient hepatocytes. Taken together, IRF-1 mediates IFN-gamma signaling into primary hepatocytes for cell cycle arrest via p53 expression and for apoptosis.