COS7 cells and other cell lines expressing the SV40 large T-antigen, have been of great benefit in transfection studies using transient expression vectors which contain the SV40 origin of replication, as these cells allow plasmid replication to a high copy number. As no T-antigen expressing cell line derived from a well characterised hepatocyte-like continuous cell line currently exists, the establishment of such a cell line for studies which require expression of hepatocyte-specific factors would be extremely useful. A HepG2-derived stable cell line (THT1) was therefore developed which demonstrates a high level of transfection efficiency whilst retaining hepatocyte-like features, such as the production of hepatitis B virus. The THT1 cell line displayed chromosomal integration of the SV40 T-antigen gene and nuclear expression of the antigen. The cell line also maintained the general morphological features of its parent cell line, and showed an increased rate of cell growth.