Alpha-fetoprotein (AFP) was detected, by Ouchterlony immunodiffusion technique, in 81.5% of patients with histologically confirmed diagnosis of hepatocellular carcinoma. The test gave negative results with 35 cases of acute viral hepatitis, 7 haemochromatosis, 6 micronodular cirrhosis and 2 cholangiocellular carcinoma. Curiously, one patient with postnecrotic cirrhosis, a well recognized sequela of viral hepatitis, whose liver cell regeneration also showed "atypical changes", was AFP positive. AFP was not detected in sera from the general population which comprised 1029 male blood donors, 144 antenatal and 106 maternity cases. The only exception was the case of a woman who aborted a 5-month old foetus. A follow-up serum sample taken 3 months later was, however, negative for AFP. The frequency of hepatitis B surface antigen (HBsAg) detection in patients with hepatocellular carcinoma (25.9%) was 4 to 5 times higher than that in the general population. This strong association between HBsAg and primary liver cancer in countries where liver tumours are often AFP secretors suggests a role for hepatitis B virus, not only in the aetiology of the cancer, but also in the reactivation of the gene encoding this foetal protein.