The distribution and quantitative expression of HBcAg in relation to serum HBeAg and liver histology before and after a trial of interferon in 50 patients with chronic type B hepatitis were evaluated using polyclonal and monoclonal antibodies. In general, both antisera showed a similar pattern in terms of the distribution of HBcAg, with predominant localisation of HBcAg in the cytoplasm in HBeAg positive patients with chronic active hepatitis. Semiquantitative analysis showed, however, that there was a higher degree of cytoplasmic expression of HBcAg with polyclonal than with monoclonal anti-HBc. Some of the HBeAg positive patients with only a focal expression of HBcAg in the cytoplasm by polyclonal anti-HBc showed no expression of HBcAg with monoclonal anti-HBc. The expression of HBcAg with polyclonal anti-HBc correlated better with the histological features of chronic active hepatitis or the persistence of serum HBeAg on follow up, suggesting that it did not result from non-specific or false positive staining. All of the HBeAg negative patients with minimal histological changes or inactive cirrhosis were HBcAg negative with both antisera. In conclusion, though both polyclonal and monoclonal antibodies produced a quite similar distribution of HBcAg in patients with chronic type B hepatitis, polyclonal antibody seemed to be more sensitive in detecting HBcAg in the cytoplasm than did monoclonal anti-HBc, and the expression of HBcAg with polyclonal anti-HBc correlated better with the clinical and histological outcome.