1. Purified ovomucin was isolated as an insoluble glycoprotein complex from thick egg white. 2. A homogeneous glycoprotein, designated alpha-ovomucin, of molecular weight 210000 and containing N-acetylglucosamine (6.7%, w/w), N-acetylgalactosamine (0.6%, w/w), galactose (1.8%, w/w), mannose (4.6%, w/w), N-acetylneuraminic acid (1.0%, w/w) and sulphate (0.7%, w/w), was isolated from preparations of reduced ovomucin by sedimentation equilibrium in a density gradient of caesium chloride formed in the presence of 4m-guanidine hydrochloride. 3. A carbohydrate-rich fraction, designated beta-ovomucin (which is homogeneous by sedimentation-velocity analysis in 5m-guanidine hydrochloride but which is heterogeneous by analytical sedimentation equilibrium in a density gradient of caesium chloride in the presence of 4m-guanidine hydrochloride), containing N-acetylglucosamine (11.0%, w/w), N-acetylgalactosamine (8.7%, w/w), galactose (19.2%, w/w), mannose (4.1%, w/w), N-acetylneuraminic acid (13.8%, w/w) and sulphate (2.7%, w/w), was also obtained from preparations of reduced ovomucin by the density-gradient method. 4. Mild acid hydrolysis of the unfractionated ovomucin complex showed that N-acetylneuraminic acid occupied a terminal position of the oligosaccharide chains. 5. Alkaline beta-elimination reactions with the unfractionated ovomucin complex indicated that N-acetylgalactosamine was linked by alkali-labile bonds to hydroxy amino acids.