The structure of the carbohydrate units of alpha-fetoprotein from fetal calf serum has been studied. Glycopeptides and oligosaccharides were prepared from alpha-fetoprotein by protease treatment and hydrazinolysis, respectively, and subjected to carbohydrate and amino acid analysis. Two N-glycosidic glycans are present in each alpha-fetoprotein molecule. These were separated into concanavalin A (ConA)-reactive and nonreactive species on ConA-Sepharose. Methylation analysis, Smith degradation, sequential exoglycosidase treatments, and sizing suggested that the major, ConA-nonreactive fraction is composed of triantennary and the minor, ConA-reactive fraction, of biantennary complex-type ConA-reactive and -nonreactive fractions of intact alpha-fetoprotein, respectively, and refractionated on Con A-Sepharose. The results indicate that 75% of alpha-fetoprotein molecules contain two triantennary complex-type glycans, 20% contain one triantennary and one biantennary glycan, and 5% contain two biantennary glycans. The last two molecular variants are bound to ConA. These results explain, at least in part, the previously found heterogeneity of alpha-fetoprotein with respect to charge and molecular size, and provide a biochemical basis for the differing reactivities toward ConA of alpha-fetoprotein from the yolk sac, fetal liver, and various tumors.