ABSTRACTA high-molecular-weight glycoprotein (HMG), representing the majority of the soluble glycoprotein hexosamine and hexose in the intestine, was isolated by Sepharose 4B chromatography from high-speed supernatants of rat small intestinal homogenate. Fluorescein-labelled globulin, from rabbit antiserum produced against HMG, specifically stained supra-nuclear mucus vesicles of goblet cells in small intestine and colon as well as gastric pit cells in the body of the stomach and mucus-producing cells in the sublingual salivary gland. A single precipitin line was found when HMG was tested against its antibody by agar immunodiffusion and immunoelectrophoresis. No cross-reactivity was observed between antibody and rat serum or extracts from microvillus membrane, human colon and pig intestine. Precipitin lines which fused with the HMG precipitin arc in apparent identity were observed with antigens in intestinal lumen washings, and in small-molecular-weight fractions from intestinal cell sap. When studied by cellulose acetate electrophoresis, cetyl trimethylammonium bromide precipitation and precursor labelling with [1-14C]glucosamine, HMG behaved as a single homogeneous glycoprotein free of detectable protein contamination. These results imply that HMG is a major component of goblet-cell mucus in the small intestine, and suggest that it is similar to mucin produced throughout the gastrointestinal tract. HMG is the first glycoprotein, isolated without the aid of proteolytic agents, which has been specifically identified as a product of the goblet cell.