AbstractTo obtain information on the relationship between a carcino‐fetal protein, α‐fetoprotein, fetal proteins from other species and other human serum proteins including the fetoprotein in patients with hepatocellular cancer, α‐fetoprotein was purified and characterized from the serum of human fetuses. The protein is composed of a single polypeptide chain. A molecular weight of 70,000 was obtained by gel electrophoresis in the presence of SDS and by gel filtration. The isoelectric point of α‐fetoprotein was found to be 4.75. It contains 4.3% carbohydrate with 2 moles of sialic acid per mole of protein. The amino acid composition of α‐fetoprotein was determined.