Benzyl 2-acetamido-3, 4-di-O-acetyl-2-deoxy-alpha-D-galactopyranoside was condensed with 2, 3, 4-tri-O-benzyl-6-O-p-nitrobenzoyl-alpha-D-glucopyranosyl bromide or with 2, 3, 4-tri-O-benzyl-6-O-p-methoxybenzoyl-alpha-D-glucopyranosyl bromide in benzene at 50 degrees in the presence of pyridine, to give benzyl 2-acetamido-3, 4-di-O-acetyl-2-deoxy-6-O-[2, 3, 4-tri-O-benzyl-6-O-p-nitro(or methoxy)benzoyl]-alpha-D galactopyranoside in excellent yield. The title disaccharide was obtained in crystalline form after deacylation and catalytic hydrogenation. It proved identical with a disaccharide isolated from Salmonella johannesburg 5.58 (40) converted by phage phi 1 (40). In order to bind this disaccharide covalently onto various proteins, p-aminophenyl 2-acetamido-2-deoxy-6-O-(alpha-D-glucopyranosyl)-alpha-D-galactopyranoside has been obtained in an analogous way, starting from p-nitrophenyl 2-acetamido-3, 4-di-O-acetyl-2-deoxy-alpha-D-galactopyranoside.