Human chorionic gonadotropin (hCG) highly purified from urine of the patient with choriocarcinoma contains four asparagine-linked sugar chains in one molecule. The sugar chains were quantitatively liberated as radioactive oligosaccharides from polypeptide portion by hydrazinolysis followed by N-acetylation and NaB3H4 reduction. The structures of these sugar chains were determined by the combination of sequential glycosidase digestion, periodate oxidation, and methylation analysis. As compared with the sugar chains of normal urinary and placental hCG reported previously, they include several prominent structural differences. More than 97% of the sugar chains of choriocarcinoma hCG was free from sialic acid, while the sugar chains of normal hCG were mostly sialylated. Choriocarcinoma hCG contains unusual biantennary complex-type sugar chains in addition to regular tri-, bi-, and monoantennary sugar chains. These sugar chains have two outer chains linked at the C-2 and C-4 positions of the same alpha-mannosyl residue of the trimannosyl core. Since normal hCG does not contain any triantennary sugar chains, occurrence of Gal beta 1 leads to 4GlcNAc beta 1 leads to 4Man alpha 1 leads to group is another characteristic feature of the sugar chains of choriocarcinoma hCG. The evidence that the monoantennary sugar chain of Man alpha 1 leads to 6(Gal beta 1 leads to 4GlcNAc beta 1 leads to 2Man alpha 1 leads to 3)Man beta 1 leads to 4GlcNAc beta 1 leads to 4(Fuc alpha 1 leads to 6)GlcNAc leads to Asn is not found in normal hCG and the sum total of fucosylated sugar chains is 50%, which is twice as much as normal hCG, indicated that fucosylation is also modified in choriocarcinoma tissue.