Intercellular adhesion molecule 1 (ICAM-1), a counter-receptor for lymphocyte function-associated antigen 1 on T cells, is critically important to a wide variety of adhesion-dependent leukocyte functions, including antigen presentation and target cell lysis. ICAM-1 expression by hepatocytes is increased in areas of inflammation and necrosis during chronic hepatitis B. Whether induction of ICAM-1 is due to the effect of inflammatory cytokines or involves a direct effect of the hepatitis B virus (HBV) remains unknown. In the present study, transfection of the HBV genome into human hepatoma cell lines resulted in enhanced expression of ICAM-1 protein and RNA in the absence of inflammation. Results of subgenomic transfections indicated that the HBV X protein (pX) induced ICAM-1 expression. Nuclear run-on assays showed that pX induced the ICAM-1 gene by increasing its rate of transcription. Although both pX and interferon gamma induced transcription of ICAM-1, addition of interferon gamma to cells expressing pX did not show an additive or synergistic effect. These results indicate that pX can directly regulate expression of ICAM-1 and may participate in the immunopathogenesis of HBV infection.