The structure of an antigenic, sulfated glycoprotein from chick chorioallantoic fluid has been investigated by exogalactosidase digestion, methylation and mass spectral analyses, periodate oxidation, and Smith degradation. The main carbohydrate chains are composed of D-galactosyl residues linked at C-3 and 2-acetamido-2-deoxyglucose residues linked at C-4. Fucose and N-acetylneuraminic acid residues are nonreducing terminal groups, and the N-acetylneuraminic acid groups are linked to the D-galactose residues at C-3. Most of the sulfate groups (91% of the sulfate) are located on C-6 of the 2-acetamido-2-deoxyglucose residues, and the rest on C-6 of the D-galactose residues. A large number of the D-galactose residues (36.9% of the total) are present as nonreducing terminal groups and another 21.7% of the D-galactose residues are in penultimate position to the nonreducing terminal N-acetylneuraminic acid residues. Although mild periodate oxidation indicates the presence of D-galactose in furanoside form (5.5% of total D-galactose), no 5-O-methyl derivative of D-galactose was observed on methylation.