Molecular species of sialic acids released by mild acid hydrolysis from mucins of bovine small and large intestine were characterized by two dimensional thin-layer chromatography and gas chromatography-mass spectrometry. The small and large intestinal mucins contained both N-acetyl- and N-glycolylneuraminic acid. Although O-acetylated derivatives were not detectable in the small intestinal mucin, 9-O-acetyl-N-glycolylneuraminic acid was demonstrated in the large intestinal mucin by thin-layer chromatography and its structure was confirmed by mass spectrometry. Moreover, the presence of three kinds of O-acetyl-N-acetylneuraminic acid and five kinds of O-acetyl-N-glycolylneuraminic acid in the large intestinal mucin was suggested by thin-layer chromatography and mass chromatography. The limited distribution of O-acetylated neuraminic acids suggests that their derivatives are only produced in the mucous glands of the large intestine.