Glycoproteins containing heavily O-glycosylated, mucin-like domains serve important biological functions in which the O-linked glycans play a major role. Although not well understood, O-glycan structures are known to vary reproducibly as a function of their position in the peptide sequence. Toward understanding such behavior, an analysis of the in vivo Core 1 (beta-Gal(1-3) alpha-GalNAc-O-Ser/Thr) site-specific glycosylation pattern of the porcine salivary gland mucin 81 residue tandem repeat has been undertaken. When a kinetic modeling approach is utilized, the in vivo Core 1 glycosylation pattern could be reproduced by incorporation of the inhibitory effects of neighboring residue glycosylation plus and minus three residues of the site of glycosylation. The obtained positional weighing parameters suggest that the porcine salivary gland Core 1 transferase (UDP-galactose:glycoprotein-alpha-GalNAc beta3-galactosyltransferase) is most sensitive to the presence of glycans C terminal to the site of glycosylation. The analysis further suggests that neighboring peptide core alpha-GalNAc residues are primarily responsible for the effect. These findings further support the notion that the formation of the Core 1 structure, an important initial step in O-glycan biosynthesis, may be regulated to a large extent by neighboring residue glycosylation. As a result, the development of approaches for predicting O-glycan core structures in a site-specific manner may now appear a distinct possibility.