The major glycopeptides purified from the tryptic digests of M and N blood-group glycoproteins were degraded with cyanogen bromide into two fragments. The chemical composition and serological activities of the fragments obtained were determined. The results show that M and N blood-group determinants are located on the smaller N-terminal fragments, containing 8 amino acid residues and only alkali-labile oligosaccharide chains. Two of 8 amino acid residues are different in M-specific and N-specific glycopeptide. All glycopeptides obtained inhibited Vicia graminea anti-N lectin, but the N-terminal fragment of N-glycopeptide was a better inhibitor than others. Treatment with neuraminidase or acetylation of amino groups destroyed the M and N blood-group activity and increased the activity towards Vicia graminea anti-N lectin of all glycopeptides studied.