The serum alphafetoprotein level (AFP) was studies in 125 histologically verified cases of hepatocellular carcinoma, 66 other malignancies, 74 cases of cirrhosis of the liver, 60 of chronic aggressive hepatitis, 12 of chronic persistent hepatitis, 16 of subacute hepatitis, 36 of acute viral hepatitis, and 13 healthy hepatitis B-surface antigen (HBsAg) carriers. Double immunodiffusion and radioimmunoassay (RIA) were used in all cases. AFP greater than 10 ng-ml appeared in 90% of the cases, and was above 400 ng/ml in 69%. In 80% of those above 400 ng/ml, AFP could also be demonstrated by immunodiffusion. The AFP level in hepatocellular carcinoma was discovered to decline as the age increased. It also appeared to be related to the tumor cell type; the relatively immature cell type was more frequently associated with a higher AFP level. The presence of HBsAg did not influence the AFP level. Although the AFP in other malignancies and liver diseases ranged abnormally from 14 to 69%, the level did not exceed 400 ng/ml as in our cases of hepatocellular carcinoma (except in one case). Thus, this figure provides a diagnostic serum level of AFP for the identification of hepatocellular carcinoma.