Small interfering RNA (siRNA) is a powerful tool to silence gene expression in mammalian cells including genes of viral origin. To evaluate the therapeutic efficacy of siRNA against the hepatitis B virus (HBV), we studied the effect of transfection of the HBV-inducible cell lines HepAD38 and HepAD79 with siRNA specific for the core gene of the HBV genome. HepAD38 cells produce wild-type HBV, whereas HepAD79 cells produce the lamivudine resistant YMDD variant. Transfection of HepAD38 cells with either 1.6 or 4 microg/ml siRNA resulted in a profound inhibition (72% and 98%, respectively) of viral replication (as assessed by real-time quantitative PCR). The inhibitory effect was corroborated by a marked reduction of HBV core protein synthesis in induced HepAD38 cells. In HepAD79 cells, transfected with 1.6 or 4 microg/ml HBV-specific siRNA, virus production was reduced by 75% and 89%, respectively.