Viridans streptococci that participate in the microbial colonization of teeth have cell wall polysaccharides composed of linear phosphodiester-linked hexa- or heptasaccharide repeating units, each containing a host-like disaccharide motif, either Gal beta 1-->3GalNAc or GalNAc beta 1-->3Gal. Whereas strains with GalNAc beta 1-->3Gal-containing polysaccharides co-aggregated with streptococci that possess GalNAc-sensitive lectins, strains with either host-like motif co-aggregated with Actinomyces spp. The latter interactions reflected the specificity of Actinomyces spp VSports手机版. lectins for common features of Gal beta 1-->3GalNAc and GalNAc beta 1-->3Gal. Thus, alpha-linked glycosides of both disaccharides were much more potent inhibitors of co-aggregation than Gal or GalNAc. Six non-bacterial lectins also reacted with the streptococcal polysaccharides. In general, precipitation of each lectin with each polysaccharide involved binding of Gal or GalNAc within the host-like motifs, but not saccharides outside these regions. The lectins of Ricinus communis, Abrus precatorius, Codium fragile and Agaricus bisporus were most reactive with the Gal beta 1-->3GalNAc-containing polysaccharides, the Wisteria floribunda lectin with the GalNAc beta 1-->3Gal-containing polysaccharides and the Bauhinia purpurea lectin with polysaccharides containing either disaccharide. Thus, lectin recognition of the streptococcal cell wall polysaccharides involved either the common or specific sides of the Gal beta 1-->3GalNAc and GalNAc beta 1-->3Gal motifs present within these molecules. .