Hydrolysis of the Bordetella pertussis endotoxin, extracted from both "phase I" and "phase IV" bacteria, with 4 M HCl for 1 h at 100 degrees C, released the disaccharide named in the title; it was isolated by paper electrophoresis or by ion-exchange chromatography in about 1% yield (w/w). The structure of the heptose could be rigorously established by chemical degradation; the facts that the glucosaminidic linkage was hydrolysed by an enzyme preparation containing both, alpha and beta-N-acetylglucosaminidase activities, whereas it was resistant to cleavage by pure beta-N-acetylglucosaminidase strongly support the assumption that the disaccharide contains an alpha-D-glucosaminide linkage.