The transglycosylation activity of Arthrobacter endo-beta -N-acetylglucosaminidase (Endo-A) was used for the enzymatic synthesis of a novel oligosaccharide, Man(6)GlcNAc-5-bromo-4-chloro-3-indolyl-beta -glucoside (Man(6)GlcNAc-Glc-beta -X). Various endo-beta -N-acetylglucosaminidases hydrolyzed this oligosaccharide, producing Man(6)GlcNAc and Glc-beta -X. The E. coli strains coexpressing Endo-A and beta -glucosidase formed blue colonies in the presence of Man(6)GlcNAc-Glc-beta -X. Therefore, endo-beta -N-acetylglucosaminidase activity could be directly detected by the plate assay. This simple plate assay is useful for screening microorganisms for endo-beta -N-acetylglucosaminidase activity.