Phage G13 binds to the carbohydrate part of lipopolysaccharides from rough mutants of Salmonella and Escherichia coli as the first event of infection. Equil. dialysis inhibition studies with native and synthetic trisaccharides as inhibitors suggested that phage G13 recognizes branched oligosaccharides having 6-O-a- or 7-O-a-glycosyl groups with a-Man(1->3)[a-Man(1->6)]Man (Man[Man]Man) and a-Glc(1->3)-[a-Hep(1->7)]a-Hep(1->3)a-Hep(1->5)Kdo as the smallest saccharides with inhibitory activity. Of four synthetic analogs to Man[Man]Man only Man(1->3)[a-Gal(1->6)]a-Man-OMe (Man[Gal]Man) and a-Glc(1->3)[a-Hep(1->7)]a-Hep-OMe (Glc[Hep]Hep) inhibited the binding of labeled E. coli C core nonasaccharide ligand to G13 with activities which were 10- and 15-fold lower than Man[Man]Man. The trisaccharides a-Man(1->3)[a-Glc(1->6)]a-Man-OMe (Man[Glc]Man) and a-Man(1->3)[a-Tal(1->6)]a-Man-OMe (Man[Tal]Man) showed no inhibition at concns. 75-fold higher than Man[Man]Man. Min. energy conformation calcns. of the saccharides using the GESA method showed that the 6-O-a-Man group in Man[Man]Man and the 7-O-a-Hep group in SL805 pentasaccharide expose their OH-2 and OH-3 groups in a similar way and these are postulated to be key structural features for binding activity. The importance of hydroxy groups at certain positions is implied from the fact that both manno- and galacto-isomers are active. It was also concluded that the O6-C6-C5-O5-C1 region of the 3-O-a-glycosyl group in the Man[Man]Man trisaccharide, or part of it, is important for the G13 binding activity. [on SciFinder (R)]