Transactivation of viral and host genes expression by hepatitis B virus X protein (HBx) is believed to be involved in hepatocarcinogenesis. The interaction of HBx with the tumor suppressor p53 and its inhibitory effect on p53 functions have been reported recently. However, the question of whether p53 is directly involved in EIBx transactivation has not yet been addressed. In this study, we delineated the interaction sites of HBx and p53 using far-Western blotting and glutothione S-transferase-resin pulldown assays. The results indicate that the HBx-binding sites are located within the oligomerizafion and specific DNA-binding domains of p53 and that the p53-binding site was confined to a small region in the HBx transactivatton domain. Mutual interference of the transactivations by HBx and p53 was detected by CAT assays in a transient transfection system. Strikingly, transactivation by HBx was observed in the p53-negative cells, Saos-2 and Hep3B, indicating that the transactivation and the p53-inhibiting functions of HBx are mutually interfering but distinct.