Serum α-fetoprotein (AFP) from cord blood and from patients with hepatitis, cirrhosis, hepatocellular carcinoma, gastrointestinal tumors and yolk sac tumor was analyzed by extended agarose gel electrophoresis coupled with our sensitive detection method of antibody-affinity blotting. AFP was separated into AFP-A, AFP-B and AFP-C from the anode to the cathode, corresponding to disialo-, monosialo- and asialo-AFP, respectively, as revealed by the results of neuraminidase digestion of serum AFP. AFP-Af, which migrated ahead of AFP-A as band or leading smear and varied widely in intensity, was eliminated in calculating the proportions of other band intensities. Disialo-AFP was the major component and mono-sialo-AFP the minor one in benign conditions, the latter being 4.2 ± 6.0% in chronic hepatitis and 9.0 ± 8.9% in cirrhosis. Monosialo-AFP in hepatocellular carcinoma was increased significantly, the proportion being 29.9 ± 11.2%. AFP in other malignancies was further characterized by the appearance of asialo-