The binding specificities of antibodies directed against the Salmonella sero-group A-specific O-antigen 2 determinant were characterized by precipitation-inhibition and enzyme-linked immunosorbent assay inhibition tests. Two different antigen O-2-specific antisera were investigated: one conventional factor O-2 serum (elicited by whole heat-killed Salmonella paratyphi A bacteria) and another elicited by the synthetic disaccharide 3-O-a-paratopyranosyl-D-mannopyranosyl (formula: see text) covalently linked via a p-isothiocyanatophenyl aglycon to bovine serum albumin (PM-BSA). The inhibition data showed that factor O-2 antibodies have combining sites which recognize structures larger than the (formula: see text) disaccharide and equal to or smaller than an O-antigen O-2-specific octasaccharide derived from the S. paratyphi A O-polysaccharide. Although the factor O-2 serum exhibited a high specificity for the homologous S. paratyphi A O-antigen it still precipitated, though weakly, a heterologous Salmonella typhimurium O-antigen. In contrast, anti-PM-BSA antibodies were exclusively specific for the O-2 determinant of the native polysaccharide antigen. The combining sites of these antibodies best recognized the (formula: see text) disaccharide, including the linkage arm and the lysyl residue of the BSA carrier protein molecule. These data extend earlier findings as to the superior specificity of anti-PM-BSA antibodies as compared to conventional factor O-2 antibodies.