Expression of X protein and hepatitis B virus replication in chronic hepatitis. The X protein can act on the enhancer of hepatitis B virus in an in vitro system and elevate the transcriptional level of hepatitis B virus. However, because no relationship had been reported between X protein expression and hepatitis B virus replication in patients with chronic hepatitis B, we focused on its expression in the liver in comparison with markers of hepatitis B virus replication. Liver biopsy samples and sera from 59 carriers with HBsAg were examined immunohistochemically for X protein using rabbit IgG against recombinant X protein. There was a significant difference in the serum hepatitis B virus DNA level between X protein-positive and -negative patients (p less than 0.001). Serum pre-S1 and pre-S2 antigens were also measured quantitatively by enzyme immunoassay using monoclonal antibodies specific against each antigen. The titers of pre-S1 antigen in patients positive for X protein were significantly higher (p less than 0.001) than those of the X protein-negative patients (3.02 +/- 0.99 vs. 2.00 +/- 0.59, respectively). Similarly, the titers of pre-S2 antigen were 2.98 +/- 0.91 vs. 1.94 +/- 0.54, respectively (p less than 0.001). The rate of positivity of the X protein was higher (38 of 49; 77.6%) in the replicative group (serum HBeAg, serum hepatitis B virus DNA or HBcAg in liver positive) compared with that observed in the nonreplicative group (3 of 10; 30%--serum HBeAg, serum hepatitis B virus DNA and HBcAg in liver negative) (p less than 0.01). 