The glycoprotein of human gastric mucus has been isolated and purified from mucous gel scraped from the surface of resected mucosa. All samples of mucus when analyzed by gel filtration were found to contain native, gel-forming, polymeric glycoprotein, together with varying amounts of the lower-molecular-weight glycoprotein. The galactosamine and glucosamine content of the lower-molecular-weight glycoprotein was the same as that for the native polymeric glycoprotein. The absence of serum glycoproteins and proteoglycans in the glycoprotein preparations was demonstrated. Gastric mucus from normal mucosa obtained by resection of the antrum during pancreatoduodenectomy, from duodenal ulcer patients, and from gastric ulcer patients contained 33.4% +/- 5.1%, 50.2 +/- 3.3%, and 65.1 +/- 2.8%, respectively, of the lower-molecular-weight glycoprotein. These results show the total mucous gel of the gastric ulcer group [and to a lesser extent, of the duodenal ulcer group] contains more lower-molecular-weight mucous glycoprotein, which from previous studies in vitro would suggest a weaker mucous gel structure.