Hepatitis B virus (HBV) core gene translational products were localised previously in the cytoplasm and/or in the nuclei of infected cells. We investigated in naturally infected human hepatocytes whether this variation in the subcellular expression is due to differences in the presence of assembled core particles and other core gene derived proteins, the expression of HBeAg and the processing of liver tissue. By immunostaining of liver specimens infected with HBeAg-positive and HBeAg-minus variants of HBV, using monoclonal antibodies specific for assembled core particles and for various epitopes on denatured core protein, it was shown that virtually all immunoreactive core gene products are assembled into core particles. The latter are present both in the nuclei and in the cytoplasm of hepatocytes, independent of the infecting virus strain. A marked reduction or absence of immunoreactivity, observed with some monoclonal antibodies, was shown to result from nucleotide sequence variations within or close to the corresponding epitope. These results demonstrate that immunoreactive products, derived from the HBV core gene, in the nuclei and cytoplasm of human hepatocytes represent assembled core particles and that monoclonal antibodies with known recognition sites can reveal region-specific core gene variation of the infecting HBV population.