It has been shown that hepatitis B virus (HBV) X antigen (HBxAg) functioned as a transactivating element which can act on the enhancer of HBV in an in vitro system and elevate the transcriptional level of HBV. In this study we investigate the relationship between HBxAg expression and HBV replication in patients with chronic hepatitis B and cirrhosis. Rabbit IgG against recombinant HBxAg which was synthesized in E. coli were prepared and used for the detection of HBxAg. HBV DNA was amplified by polymerase chain reaction technique by using primers from HBx gene sequence. Liver tissue samples and sera from the patients were examined immunohistochemically for HBxAg and serologically for HBxAg/anti-HBx respectively. We focused on its expression in these samples in comparison with markers of HBV replication. It was found that in liver HBxAg was present in 72.7% of the patients with chronic active hapatitis (CAH) and 92.6% of those with cirrhosis, while the positivity rate of HBcAg in cirrhosis patients was only 47.8%. In the sera of the patients with CAH, chronic persistant hepatitis and cirrhosis HBxAg was present in 44.4%, 66.6% and 33.3% respectively. It was similar to that observed with HBeAg. Moreover in these HBxAg positive sera HBV DNA can also be detected. It was shown that higher rate of positivity of HBxAg was found in patients with replicative markers (serum HBeAg, serum HBV DNA or liver tissue HBcAg positive). Our results indicate that expression of HBxAg is closely correlated with HBV replication and HBxAg may be an important marker in chronic HBV infection.