Rat monoclonal antibody MASC1-MR9 binds to a mannan of Candida species and the O-antigenic polysaccharides of Salmonella bacteria of serogroups C1 (CO) and E (EO). Mannan and glycoconjugates comprising BSA and O-antigen polysaccharides, decasaccharide-BSA (CO-BSA) or trisaccharide-BSA (EO-BSA), inhibited each other's reactivity with MR9. The saccharides b-D-Manp-(1->6)-a-D-Manp-1-OMe, b-D-Manp(1->3)-a-D-Manp-1-OMe, b-D-Manp(1->2)-a-D-Manp-1-OMe (corresponds to the terminal non-reducing end of Salmonella serogroup C1 O-antigen) and b-D-Manp(1->4)-a-L-Rhap(1->3)-a-D-Galp-1-O-p-trifluoroacetamido aniline (corresponds to the backbone of Salmonella serogroup E O-antigen) inhibited the binding of MR9 to these antigens whereas a-D-Manp(1->3)-a-D-Manp-1-OMe and a-D-Manp(1->4)-a-L-Rhap-1-O-p-nitrophenyl did not. Saccharides (3-10 residues) of mammalian origin with terminal and internal Manpa-1->2, Manpa-1->3 and Manpa-1->6 residues also failed to inhibit at any concn. None of the saccharides with internal b-mannosyl residue was able to inhibit the MR9 antibody. Monosaccharides D-mannose, b-D-Manp-1-OMe and 1,5 anhydro-D-mannitol inhibited the MR9 monoclonal antibody whereas a-D-Manp-1-OMe, b-D-Glcp-1-OMe, and b-D-Galp-1-OMe did not. In addn. a Klebsiella K24 capsular polysaccharide contg. a b-D-Manp(1->4)-a-D-GlcA (GlcA, glucuronic acid) as a structural element possessed an inhibitory activity. MR9 therefore recognizes an epitope within b-mannose monosaccharide residues at the terminal non-reducing ends of carbohydrate chains in mannan, and polysaccharides in Salmonella serogroups CO and EO and Klebsiella K24. [on SciFinder (R)]