4P-X (beta-D-galactopyranosyl-(1 ->4)-beta-D-galactopyranosyl-(1 ->6)-[beta-D-galactopyranosyl-(1 ->4)]-beta-D-glucopyranose) is included in galacto-oligosaccharides (GOSs) produced by beta-galactosidase derived from Bacillus circulans. 4P-X has been known to induce particularly strong allergies. High purity 4P-X is essential for use as a standard to quantify the amount of 4P-X in GOSs; however, the isolation of high purity 4P-X has never been reported. In this study, we achieved the synthesis of 4P-X by a combination of organic and enzymatic chemical syntheses in a short time. This is the first report of isolated, high purity 4P-X. (C) 2017 Elsevier Ltd. All rights reserved.