The functional effect of the interaction of E2F1 and hepatitis B virus X protein (HBx) on the promoter of human p53 gene was studied using chloramphenicol acetyl transferase (CAT) assay. E2F1 activated the p53 promoter through E2F1 binding site. As previously reported, HBx repressed the p53 promoter through E-box. When E2F1 was cotransfected with HBx, E2F1 overcame the repressive effect of HBx on the p53 promoter through the E2F1 site. However, in the thymidine kinase (tk) heterologous promoter system with the E2F1 binding sites, cotransfection of E2F1 and HBx showed a strong synergistic activation. An in vitro interaction assay showed that E2F1 and HBx physically bind with each other. Analyses of the interaction domain with the GAL4 fusion protein showed that the pRb-binding domain of E2F1 was necessary for the functional interaction of these two proteins. Taken together, these results imply the functional inhibitory action of E2F1 on the HBV life cycle and HBV-mediated hepatocellular carcinogenesis (HCC). Therefore, the normal or enhanced function of E2F1 gene would be important in controlling the HBx function in HCC.