Mycophenolate mofetil (MMF), the morpholinoethyl ester of mycophenolic acid (MPA), is currently used as an immunosuppressive agent in kidney transplant recipients. After oral administration, MMF is hydrolysed to MPA, the active compound, which is a potent inhibitor of inosine monophosphate dehydrogenase (IMP-DH). Inhibition of this enzyme results in a depletion of the intracellular GTP and dGTP pools. MPA has been shown to inhibit the replication of a number of viruses, including arena viruses (Junin and Tacaribe), yellow fever virus, reovirus-1, parainfluenza-3 virus, Coxsackie B4 virus, Epstein–Barr virus and human immunodeficiency virus. To examine whether MPA also has an inhibitory effect on HBV replication, experiments were performed using cultures of primary human hepatocytes and HBV-transfected, HepG2 2.2.15 cells. After in vitro infection with HBV in human hepatocytes, HBV covalently-closed-circular (ccc) DNA and HBV mRNAs were detectable in the cells during the 10 days following infection. HBV DNA and hepatitis B surface antigen (HBsAg) were also secreted into the culture medium. In the presence of 10 μg ml–1 MPA (the therapeutic serum level of MPA as an immunosuppressive agent) in culture medium, HBV ccc DNA and HBV mRNAs became undetectable 5 days after treatment was started. The secretion of HBV DNA and HBsAg into the medium was also markedly reduced. No cytotoxic effect of the drug was noted during the experiments. The effect of MPA on HBV replication was abolished by the presence of guanosine (50 μg ml–1). In HepG2 2.2.15 cells (which contain an integrated tandem dimer of the HBV genome), MPA treatment had no significant inhibitory effect on the secretion of HBV DNA and HBsAg into the culture medium. HBV ccc DNA and HBV mRNAs in HepG2 2.2.15 cells were also not affected. The observed effect of MPA on HBV replication in primary human hepatocyte cultures may involve only episomal replication and may have clinical implications, especially before integration of HBV DNA into the host genome.