Three murine, monoclonal antibodies, IgM 5286 F2, IgM 5297 C1, and IgG 5338 H4 were generated against Shigella dysenteriae type 1 O-specific polysaccharide (O-SP)-conjugate. They are specific for the O-SP, which is a poly-[alpha-L-rhamnopyranosyl-(1-->3)-alpha-L-rhamnopyranosyl-(1-->2)-al pha-D-galactopyranosyl-(1-->3)-2-deoxy-2-amino-N-acetyl-alpha-D-glucopyr anosyl]. The VH and VL genes of these antibodies were cloned and their sequences determined. They showed 93% homology, but were quite different to the primary sequence of IgM 3707 E9, of the same O-SP-specificity, previously reported. The fine-specificities of both IgG 5338 H4 and IgM 3707 E9 were for the same disaccharide moiety in the O-SP, while IgMs 5286 F2 and 5297 C1 showed fine-specificity for the entire repeating unit of the O-SP. Therefore, divergent sequences can confer upon antibodies similar-, or even identical-carbohydrate-epitope fine-specificity. In addition, close primary sequence-homology does not preclude differences in antibody fine-specificity.