Tryptic digests of ovine submaxillary apomucin were fractionated by gel filtration and ion exchange chromatography to give 14 peptide fractions. Three purified tryptic peptides, representing 106 of the 650 residues in apomucin, were submitted to automated sequence analysis. The NH2-terminal 50 of the 74 residues in one peptide and the entire sequence of the other two hexadecapeptides were established. These studies suggest that purified ovine submaxillary, mucin is chemically homogeneous, containing a unique primary structure without substantial repeating sequences in its polypeptide chain. The sequences adjacent to 28 known O-glycosidically substituted seryl and threonyl residues were compared. No homologies were apparent around the glycosylated seryl and threonyl residues which might define the specificity of the UDP-N-acetylgalactosaminyl:mucin polypeptide transferase that incorporates N-acetylgalactosamine into O-glycosidic linkage in glycoproteins. However, there appears to be a minimum size requirement for glycosylation, because the transferase catalyzes glycosylation of tryptic peptides efficiently, while chymotryptic and thermolytic peptides were much poorer substrates for the transferase.