The frequency distribution of HBs Ag in different parts of the world reveals a relatively high frequency among healthy members of population groups inhabiting areas of high incidence of liver cell carcinoma. Similar high frequencies of HBs Ag are also found in those areas where macronodular cirrhosis is relatively common and is usually complicated by liver cell carcinoma. In geographic areas with low incidence of liver cell carcinoma and macronodular cirrhosis, a relatively low frequency of HBs Ag is usually encountered in the population. The frequency of HBs Ag is relatively higher in patients with liver cell carcinoma with or without cirrhosis than in comparable controls. The subtypes of the antigen do not correlate with the incidence of liver cell carcinoma and there is also no correlation between alpha fetoprotein and HBs Ag in the presence of liver cell carcinoma. HBs Ag is very rarely detected in patients with micronodular cirrhosis or in liver cell carcinoma which may be its complication. It would appear that HBs Ag is necrogenic in the liver and is capable of producing hepatic necroses or hepatitis which may progress to macronodular cirrhosis. The areas of hepatic necroses may either progress to liver cell carcinoma or the resultant macronodular cirrhosis may be complicated by carcinoma. The oncogenic potential of HBs Ag requires further studies.