Glycophorin A was digested with glycoprotease (Pasteurella haemolytica) and the digest was fractionated by a combination of high-pressure column chromatographies to produce the glycopeptides GPA-1 to GPA-6. Sequence analysis of the glycopeptides revealed that two serine residues (Ser-14 and Ser-15) are not glycosylated, Thr-17 and Ser-19 being glycosylated instead, in disagreement with the accepted structure. The glycopeptides thus obtained were treated with sialidase and beta-galactosidase. The Tn antigenicity, as assayed by the binding to a monoclonal anti-Tn antibody (MLS 128), was found exclusively in the glycopeptides including three (cluster I) or four (cluster II) consecutive residues of GalNAc-Ser/Thr, whereas the glycopeptide (GPA-2) containing two nonconsecutive GalNAc-Ser/Thr residues had practically no Tn antigenicity. The immunoreactivities of GPA-1 and GPA-3, containing both clusters I and II, and GPA-4, containing cluster II, were 63% (calcd. 67%), 81% (calcd. 86%), and 50% (calcd. 50%), respectively, of the immunoreactivity of GPA-5 or GPA-6, containing cluster I (the average being taken as the basis), based on the reactivity per GalNAc residue. These results indicate that clusters I and II react with the antibody to the same extent. The structure consisting of three consecutive glycosylated Ser/Thr residues may be essential for Tn antigenicity in the light of previous results for ovine submaxillary mucin.