There are many similarities in structure and properties between the human hepatitis B virus and duck hepatitis B virus (DHBV). This makes DHBV infection in ducks a very useful experimental model of human HBV infection, particularly as HBV cannot be grown readily in cell culture. Ducks infected with DHBV at birth become chronic carriers of the virus, although they may not develop overt hepatitis. Some DHBV-infected ducks have been reported to develop hepatocellular carcinoma, a tumor strongly linked etiologically in humans with chronic hepatitis B infection. New antiviral immunomodulatory agents are being assessed for their ability to suppress DHBV replication in ducks as a screening test for new effective therapies for chronic type B hepatitis in man.