The establishment of stable cell lines expressing the hepatitis C virus (HCV) core protein may be important for studies of HCV pathogenesis. Human and mouse cell lines were generated expressing the HCV core protein using expression vectors driven by either the cytomegalovirus (CMV) or elongation factor-1 alpha (EF-1 alpha) promoters. Following transient transfection, HCV core protein was expressed in all cell lines. However, stable human hepatocellular carcinoma (HCC) and murine myeloma cell lines expressing the HCV core protein were only established using constructs driven by the EF-1 alpha promoter. In contrast, stable expression of the hepatitis B virus (HBV) middle envelope protein (MHBs) was obtained successfully in these cell lines using an expression vector driven by the CMV promoter. Inhibitory activity of the first 69 amino acids of the HCV core protein on the CMV promoter was found by using chimeric MHBs/HCV core protein constructs. Growth of cloned cell lines expressing the HCV core protein was slower than that of nonexpressing cell lines. However, morphological changes and cell death were not observed in the stable cell lines expressing HCV core protein. These results indicate that the HCV core protein was not directly cytotoxic to HCC and myeloma cell lines but that specific promoter elements are required to establish stable expression of the nucleocapsid structural protein.