Plasmid pHBV114 DNA, which contains 73% of the genome of hepatitis B virus (HBV), was radiolabelled with tritium to 1–2 × 10⁸ dpm/m̈g by nick translation and used as a radioactive probe to detect HBV DNA present in sections of infected liver tissue by in situ hybridisation followed by au‐toradiography. Factors affecting the sensitivity of the reaction were examined, including different methods of fixation, hybridisation time, ternperature, and buffers. The specificity of the reaction for detecting viral DNA was carefully established by the use of unrelated DNA probes, pretreatment of sections with DNAase, and comparing the stability of the binding of DNA probe at different temperatures, with the melting curve of double‐stranded DNA in solution. In the one liver studied in detail, cells containing large amounts of viral DNA were distributed in foci corresponding to areas containing morphologically damaged hepatocytes. This observation suggested a relationship between active viral replication and cell damage. Viral DNA was found mainly in the cytoplasm, although a minority of nuclei in these foci were also positive.