The binding of nineteen analogues of the upstream, terminal, monosaccharide residue of each of the O-polysaccharide (O-PS) of Vibrio cholerae O:1, serotype Ogawa and Inaba, with two murine monoclonal IgG antibodies both specific for the Ogawa LPS were measured using fluorescence spectroscopy. The use of the deoxy and the deoxyfluoro analogs allowed further refinement of the hydrogen-bonding pattern involved in the binding. Based on the binding characteristics observed for some of the ligands in the Inaba series, the binding of the monosaccharide that represents the upstream, terminal unit of the O-PS of V. cholerae O:1 serotype Inaba was redefined. We show for the first time that the upstream, terminal monosaccharide of the Inaba O-PS shows weak binding with these two anti-Ogawa antibodies. The results obtained allow further rationalization of the structural basis for the binding of V. cholerae O:1 antigens to their homologous antibodies.