The structure of the oligosaccharide repeating units of endotoxins from Gram-negative bacteria is characteristic for the different serogroups and serotypes of bacteria. Detailed examination of the cross-reactions of three enterobacterial serotypes, Proteus morganii O34, Escherichia coli O111, and Salmonella enterica sv. Adelaide O35, was performed using sensitive tests (ELISA, immunoblotting). Fine differences between the endotoxins of the bacteria were detected using silver staining of SDS-PAGE gels and chip-technology for the intact lipopolysaccharides (LPSs). The compositions of the O-specific polysaccharides of LPSs extracted from the bacteria were studied, and it was proven that the three cross-reacting bacteria contain O-antigens built from the same monosaccharides, namely colitoses linked to glucose, galactose, and N-acetyl-galactosamine. The NMR and GC-MS studies revealed that the most probable component for the cross-reaction is the rare sugar, colitose.