Hepatitis C virus (HCV) core protein has many intriguing properties as a viral factor and is implicated in cell growth regulation. In this study, the cell growth regulation potential of HCV core protein was investigated by introduction of the core genomic region into primary human hepatocytes, a natural host for virus replication and tropism. Core-transfected primary human hepatocytes displayed altered cell morphology resembling that of low-differentiated epithelial cells. Those cells retained an immortalized phenotype and exhibited continuous growth after more than 50 passages over 2 years. Stable hepatocyte transfectants exhibited albumin secretion and HCV core protein expression. Telomerase activity, a characteristic of immortalized or transformed cells, was evident in the transfected hepatocytes immediately after senescence. Anchorage-independent growth of the immortalized hepatocytes provided further evidence for a transformed phenotype. Results from these studies suggest that the HCV core protein promotes primary human hepatocytes to an immortalized phenotype, which may predispose cells over an extended period of time to undergo a transforming event. Thus, HCV core protein appears to contribute to virus-mediated pathogenesis in a persistently infected host. Copyright 2000 Academic Press.