Structural studies of the enterobacterial common-antigen, using chemical methods and fast-atom-bombardment mass spectrometry, indicate that it is a cyclic polysaccharide, composed of four, five, and, to a smaller extent, six trisaccharide repeating-units. In the structure of the antigen, given below, D-Fuc4NAc stands for 4-acetamido-4,6-dideoxy-D-galactose.