Human tumor necrosis factor alpha (TNFalpha) is a pro-inflammatory cytokine expressed in many cell types. Although the TNFalpha gene expression in human hepatocytes has been detected previously, its regulation is not well understood yet. In this study, we demonstrated that TNFalpha gene expression in human hepatoma cell line, huH2-2, was activated as a function of cell density. TNFalpha mRNA expression was low in the low-density culture, while significantly high expression was detected in the high-density culture. Moreover, stability of TNFalpha mRNA was not changed by cell density, eliminating a possibility of post-transcriptional regulation. Antibody neutralization against human TNFalpha had no significant effect on the TNFalpha mRNA expression. A cellular factor for the TNFalpha gene expression is suggested to be accumulated in the high-density cells. Data indicate that the level of TNFalpha gene transcription is elevated by a cellular factor in a cell density-dependent manner without influencing the TNFalpha secretion under the present cell-culture conditions used.