Hepatitis B virus (HBV) may be one of the agents involved in the aetiology of human primary liver cancer¹. This hypothesis is supported by (1) the similarity between the geographical distribution of chronic carriers of the viral surface antigen (HBsAg) and that of hepatocellular carcinoma (HCC); (2) the increase in the prevalence of HBV markers in serum of patients with primary liver cancer when compared with the general population¹; (3) the observation that HBV infection precedes the development of the tumour¹. Moreover, these epidemiological indications of an association between HBV infection and hepatocellular carcinoma are supported by the detection of HBV markers such as HbsAg^2,3 or viral DNA sequences, although in a non-integrated form^4,5 in tumour tissue. To study the relationship between HBV and primary liver cancer further, we looked for the presence of free or integrated viral DNA in tumour tissue of human hepatocellular carcinomas and in a HBsAg-producing human hepatoma cell line⁶. Using the blot-transfer hybridization technique and cloned HBV DNA as a probe, we have now demonstrated that the viral DNA is integrated in the cellular genome both in tumour tissue and in a hepatoma cell line.