The enterobacterial common antigen (ECA) is a carbohydrate-derived cell surface antigen present in all Gram-negative bacteria belonging to Enterobacteriaceae family. Biosynthetic pathways shared by ECA and LPS (endotoxin) suggest close connections between these antigens. ECA occurs in three different forms: a phosphatidyl-linked linear polysaccharide anchored on the cell surface (ECAPG), a cyclic form built of 4-6 repeating units localized in the periplasm (ECACYC) and as a linear polysaccharide covalently linked to LPS core oligosaccharide (ECALPS). Regardless of ECA form, poly- and oligosaccharides of ECA consist of the biological trisaccharide repeating units: →3)-α-d-Fucp4NAc-(1→4)-β-d-ManpNAcA-(1→4)-α-d-GlcpNAc-(1→, where Fucp4NAc refers to 4-acetamido-2,4-dideoxygalactose, ManpNAcA to N-acetyl-mannosaminuronic acid and GlcpNAc to N-acetylglucosamine. ECAPG and ECALPS consisting of one unit with Fucp4NAc as a terminal sugar were also identified. The number of the studies shows its occurrence in all members of enteric bacteria with a few exceptions such as Erwinia chrysanthemi. The presence of ECA was also shown for such genera as Plesiomonas [4] and Yersinia [36], previously belonging to the Vibrionaceae and Pasteurellaceae families, respectively. It was one of the reasons to include these two taxa in the Enterobacteriaceae family. The function of ECA is not fully understood, but it was reported that its occurrence is important in resistance of bacterial cells to environmental conditions, such as bile salts in the human digestive tract. The immunogenicity of ECA seems very interesting in the fact that only sparse rough Gram-negative strains, such as Shigella sonnei phase II, Escherichia coli R1, R2, R4, K-12, and Yersinia enterocolitica O:3 are able to induce the production of specific anti-ECA antibodies. It is the effect of the ECALPS, and the evidence for the existence of such covalent linkage was provided by structural analysis of S. sonnei surface antigens.