The hepatitis B virus (HBV) nucleocapsid consists of 240 viral core proteins that are arranged in a highly symmetrical structure, HBV replication can only take place inside intact nucleocapsids. In the present study, we investigated whether genetically engineered core mutants can inhibit viral replication by interfering with the formation of intact nucleocapsids. Using the duck hepatitis B virus (DHBV) model, a series of core protein mutants was generated. Polymerase chain reaction-amplified fragments from the bacterial lacZ gene expressing up to 282 amino acids were added either to the amino- or carboxy-terminus of the DHBV core protein. In addition, carboxy-terminal extensions were generated by fusing the DHBV core protein with the DHBV small surface protein or various fragments of the viral polymerase. Finally, the green fluorescent protein (GFP) was fused in-frame to the carboxy-terminus of the DHBV core protein. In this chimeric protein, GFP is still functional and can act as a reporter molecule. The various core protein mutants were tested for their potential antiviral activity by cotransfection with a replication-competent DHBV construct into the avian hepatoma cell line LMH. Carboxy-terminal, but not amino-terminal, DHBV core mutants inhibited DHBV replication by up to 90% at an effector-to-target ratio of 1:10, thus displaying a dominant negative phenotype. Antiviral activity was species-specific and caused by posttranslational interference with viral replication. The DHBV core-GFP fusion protein should be an ideal tool to assess the antiviral potential of dominant negative core proteins in vivo.