The octasaccharide Galp (Formula: see text) Rhap, the synthesized disaccharides methyl 3-O-a-tyvelopyranosyl-a-D-mannopyranoside, methyl 3-O-a-tyvelopyranosyl-beta-D-mannopyranoside and methyl alpha-tyveloside, in order of decreasing effectiveness, inhibited the precipitation of S. typhi T2 alkali-treated lipopolysaccharide by O-factor 9 serum. On a molar basis the relative inhibiting activities of the glycosides were by O-factor 9 serum. On a molar basis the relative inhibiting activities of the glycosides were 100:22:8:2. With rabbit aniserum raised against 3-O-a-tyvelopyranosyl-D-mannopyranosyl covalently linked to bovine serum albumin the relative inhibitory activities of the four glycosides were 11:100:26:3. These data establish that the 3-O-a-tyvelopyranosyl-a-D-mannopyranosyl structure is immunodominant in the Salmonella O-antigen 9. The specificity of the antigen-antibody interaction was high: glycosides in which the tyvelose (3,6-dideoxy-D-arabino-hexose) residue had been replaced by abequose (3,6-dideoxy-D-xylo-hexose) or paratose (3,6-dideoxy-K-ribo-hexose), had less than one fiftieth of the activity of the most active inhibitor in either of the two precipitation systems used. Moreover, the results show that 3-O-a-tyvelopyranosyl-D-mannopyranosyl coupled to bovine serum albumin elicits O-antibodies of higher specificity than those obtained by absorption of antibacterial immune serum.