Destruxin B, a cyclodepsipeptide was originally identified as a plant pathogen from the fungus, Alternaria brassicae. We examined the antiviral activity of destruxin B and found that it suppresses the expression of the hepatitis B viral surface antigen (HBsAg) gene in human hepatoma Hep3B cells which carry an integrated viral gene in its chromosome. In contrast, destruxin B shows no cytotoxic effect on the viability of the cells. Furthermore, it can be shown that destruxin B can reversibly suppress HBsAg production by Hep3B cells in a concentration-dependent manner with EC50 of 0.5 microM. Northern blot analysis indicates that the suppression of HBsAg gene expression by destruxin B is mainly at the mRNA level. Destruxin B not only suppresses the endogenously expressed HBsAg in the Hep3B cells but also suppresses the HBsAg produced either from the stable transfected HBV DNA in another human hepatoma HuH-7 cell line which carry no endogenous HBV genome. These results suggest that destruxin B may have future potential for development as a specific anti-HBV drug.