Woodchuck hepatitis virus (WHV) is a member of family Hepadnaviridae and closely related to hepatitis B virus (HBV). The WHV core protein (WHcAg) is a strongly immunogenic protein and forms virus-like particles. WHcAg may represent a suitable carrier system for B- and T-cell epitopes. However, the lack of a high expression system for WHcAg and defined antibodies to detect WHcAg prevents the use of this carrier system. In the present study, vectors expressing WHcAg with carboxyl-terminal truncations were constructed to determine the region of WHcAg required for assembly. The first 144 or 149 amino acid residues of WHcAg were able to efficiently assemble into particulate structures. Both truncated forms of WHcAg were accumulated in E. coli as uniform particles with a diameter of 34nm in large quantities and could be purified in milligram scale. As expected, the particles of truncated WHcAg retained the antigenicity of the full length WHcAg. However, denatured WHcAg remained to be reactive with specific antisera, suggesting that WHcAg may possess additional linear B-cell epitopes. Monoclonal antibodies against denatured WHcAg were generated and tested for their specificity. Five antibodies were found to direct the N-terminal region of WHcAg. Due to the conservation of the amino acid sequence in this region of WHcAg and HBcAg, these antibodies recognized recombinant HBcAg as well. Thus, this linear B-cell epitope is conserved on the core proteins of hepadnaviruses.