Mucus glycoproteins from the rat stomach were characterized after their isolation from homogenates of the superficial gastric mucosa by equilibrium centrifugation in CsCl density gradients. Water-soluble as well as water-insoluble glycoproteins were studied. The latter were solubilized by 2-mercaptoethanol reduction of the homogenate. From both homogenate fractions the sames two glycoproteins 1 and 2 were purified, glycoprotein 1 being present in considerably higher amount than glycoprotein 2. Their respective buoyant densities in a CsCl gradient were 1.47--1.50 g/ml and 1.56--1.58 g/ml. The two glycoproteins expressed slight differences in gel electrophoresis and gel filtration. The results from column chromatographic comparisons between reduced and unreduced glycoproteins indicated strongly that both glycoproteins 1 and 2 were built from subunits kept together by S-S bonds. The s20,w values of the reduced glycoproteins 1 and 2 were 15.7 S and 11.6 S. Glycoprotein 1 contained 5% protein, 70% carbohydrate and 1--2% sulphate, whereas these percentages for glycoprotein 2 were 10% protein, 65% carbohydrate and 10% sulphate. The molar proportions of the main sugar components galactose, fucose, glucosamine and galactosamine were 4 :2 : 4 : 1 (glycoprotein 1) and 3 : 2 : 3 : 1 (glycoprotein 2). Blood-group activity A was expressed by glycoprotein 1, whereas glycoprotein 2 showed mainly blood-group activity Leb, some B activity and also some A activity, but to a lesser extent than glycoprotein 1.