The 3,6-dideoxyhexoses are found in the lipopolysaccharides of gram-negative bacteria, where they have been shown to be the dominant antigenic determinants. Of the five 3,6-dideoxyhexoses known to A large degree of bacterial immunological diversity in gram-negative species is attributed to the portion of the lipopolysaccharides known as the 0 antigen. Exposed on the cell envelope, the 0 antigen is the dominant surface entity which is polymorphic and provides the basis for the serological classification of the family Enterobacteriaceae (4,16,26). Among the large number of monosaccharides found as components of 0-specific polysaccharides, derivatives of the 3,6-dideoxyhexoses have drawn special attention because of their highly immunogenic characteristics (22,43,45). It has been shown that only a limited number of species of gram-negative bacteria, all in the family Enterobacteraceae, are able to produce these unusual sugars, and, of the eight possible stereoisomers of 3,6-dideoxyhexoses, only five have been identified in nature. Although colitose (3, and abequose (3,6-dideoxy-D-ylo-hexose, compound I [see Fig. 1]) can be detected in the lipopolysaccharide of some Escherichia coli serotypes and Citrobacter strains, respectively, the five 3,6-dideoxyhexoses known to occur naturally are present mainly in Salmonella and Yersinia spp. as major antigenic determinants (23). For example, abequose, colitose, tyvelose (3,6-dideoxy-D-arabino-hexose, compound II), and paratose (3,6-dideoxy-D-ribo-hexose, compound III) have been found in strains of Salmonella enterica (34), while all five, including ascarylose (3,6-dideOxy-L-arabino-hexose, compound * Corresponding author. Phone: (612) 626-7541. t Present address: