To facilitate investigations of replication and host cell interactions in the hepadnavirus system, we have developed cell lines permitting the conditional replication of duck hepatitis B virus (DHBV). With the help of this system, we devised conditions for core particle isolation that preserve replicase activity, which was not found in previous preparations. Investigations of the stability of viral DNA intermediates indicated that both encapsidated DNA and covalently closed circular DNA (cccDNA) were turned over independently of cell division. Moreover, we showed that alpha interferon reduced the accumulation of RNA-containing viral particles. The availability of a synchronized replication system will permit the biochemical analysis of individual steps of the viral replication cycle, including the mechanism and regulation of cccDNA formation.