Sera from 248 patients, children and adults with various tumors, and from 484 patients with different non‐tumorous diseases, were examined for the presence of fetoprotein by immunochemical methods and specific anti‐fetal protein antiserum absorbed with serum of adult individuals. The results show that fetoprotein appears in two types of tumors only: 1) in hepatocellular carcinoma, and 2) in malignant undifferentiated teratoma. In other types of neoplasms including embryonal tumors composed of more or less differentiated tissue, as well as in cases of nephroblastoma, sympathicoblastoma, chorionepithelioma and dysgerminoma, fetoprotein was not detected in any patient. Fetoprotein can occur in the first year of life in some patients suffering from hepatitis or some other liver disease. However, its concentration is low and it has a tendency to disappear with the regression of the disease. Fetoprotein was not detected, however, in patients suffering from non‐malignant diseases, including hepatitis, after the first year of life. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company