A mucin-type glycoprotein was isolated from a human rectal adenocarcinoma, mainly be gel filtration and hydroxyapatite treatment. The glycoprotein, designated as rectal mucin-type glycoprotein (RMG), was great in amount, accounting for about 1% of the wet tissue weight. From a non-cancerous area of the patient's intestine, a similar glycoprotein reacting with anti-RMG antibodies was obtained, but the tissue content was less than 10% of the RMG content. Purified RMG contained about 70% carbohydrate in mass, and is composed of about equimolar amounts of sialic acid, galactose, N-acetylglucosamine and N-acetylgalactosamine. The polypeptide core was characterized by high contents of threonine, serine, and proline. A marked difference between RMG and the normal glycoproteins was that the sialic acid content was much higher in RMG. Of the total N-acetylgalactosamine convertible to N-acetylgalactosaminitol by reductive cleavage with alkaline borohydride, about 15% was free and the rest occupied the reducing ends of acidic oligosaccharides. The acidic oligosaccharides were fractionated into a fraction of high molecular weight and a series of oligosaccharides in which di- and trisaccharides containing sialic acid were dominant. The high molecular weight fraction contained esterified sulfate.