The complete hepatitis B virus (Dane particle) contains a circular doublestranded DNA with single stranded regions and an endogenous DNA polymerase. The HBV associated DNA polymerase closes the single stranded regions of the HBV-DNA in the presence of triphosphatase and a detergent. The DNA polymerase reaction can be inhibition by antiviral substances that exhibit different mode of actions: intercalating agents, phosphonoformiate and the triphosphates of arabinofuranosyladenine and arabinofuranosylcytosine. The value of these in vitro test for the therapy of HBsAg positive chronic liver disease is limited by the fact that is remains so far unclear whether the HBV associated DNA polymerase is a virus- or a host-coded enzyme, and whether virus- or host-coded enzymes are involved in HBV-DNA synthesis in vivo.