Duck hepatitis B virus (DHBV) - in contrast to HBV - infects, replicates and spreads in primary duck hepatocyte cultures (PDH). Therefore DHBV is used as a model system for in vitro infection. Replication deficient DHBV-mutants are widely used to study the hapadnaviral life cycle. DHBV defect mutants and recombinant (r-) DHBV are produced by cotransfection of two different plasmids. The replication deficient genome of rDHBV is reverse transcribed from the pregenomic RNA (pgRNA), which is transcribed from the DHBV-genome of a transfer plasmid. The viral proteins are expressed from a helper plasmid. Due to the production procedure, it is possible that recombinant virus stocks are contaminated with replicating wildtyp (wt) virus generated by homologous recombination which may disturb further investigations (Condray et al., 1990). To avoid this, we improved the generation of recombinant virus stocks. We used the DHBV model because it allows sensitive detection of replication competent wt virus.