The aim of this study is to investigate whether hepatitis B e antigen (HBeAg) reactivity can be detected on formalin-fixed, paraffin-embedded liver tissue, and whether immunohistochemical detection of intrahepatic HBeAg may help to distinguish between "wild-type" and "eminus" hepatitis B virus (HBV) infection. Liver biopsy specimens were analyzed from 27 patients with chronic type B hepatitis: 12 patients had serum HBeAg (group A), and 15 patients were anti-HBe positive (group B). Part of each biopsy fragment was processed for histologic and immunohistochemical studies, and a part was used for HBV-DNA analysis. Dewaxed sections from each specimen were tested with a specific monoclonal anti-HBe antibody; then a Biotin-Streptavidin kit was used as detection system. HBeAg was revealed in 10 of 12 cases of group A and in 6 of the 15 cases of group B. Pre-core region of HBV genomes, isolated from each biopsy specimen, was analyzed by direct sequencing: 10 cases of group A were found to be infected by wild-type HBV alone and 2 cases by both wild and e-minus HBV types. In group B, all the 6 cases with intrahepatic HBeAg reactivity were found to be infected by mixed viral population, whereas the 9 cases negative for such reactivity were found to be infected by e-minus HBV alone. These results show that HBeAg can be detected in formalin-fixed, paraffin-embedded liver specimens, and the method is sensitive and specific. Because the presence of HBeAg in the liver indicates a wild-type HBV infection, and the lack of detection of such antigen in the hepatocytes of anti-HBe positive subjects correlates with unmixed e-minus HBV infection, the authors conclude that this technique is a useful tool for recognizing the viral strains that infect patients with chronic type B hepatitis.