The human hepatoma cell line PLC/PRF/5 is persistently infected with hepatitis B virus. Cells cultured in vitro are known to produce the virus surface antigen but not be other structural virus proteins, the core and the e antigen. It is shown here that expression of the core antigen gene was inducible by growing the cells as a nude mouse tumour. The buoyant density in CsCl (1.31 to 1.32 g/ml) of core or e antigen produced in the tumours was very similar to that of virus core particles. Expression of the core antigen gene was shunt off by culture in vitro of core or e antigen-producing nude mouse tumour cells and induced again by subsequent passage of cells in the nude mouse. The experimental system thus allows studies on the regulation of expression of the core antigen gene.