To facilitate the routine identification of salmonellae and detailed studies of their lipopolysaccharides, we raised murine monoclonal antibodies against these organisms. We raised an immunoglobulin G1 antibody, MO2, which is specific for factor O2. By immunoblotting following sodium dodecyl sulfate-polyacrylamide gel electrophoresis, MO2 was shown to bind only the lipopolysaccharide of Salmonella paratyphi A, giving a ladderlike reaction pattern with regularly spaced reactive bands. MO2 did not react against lipopolysaccharides of other Salmonella serogroups, including those of serogroups B, C, D, E, and L. Since the lipopolysaccharides of Salmonella serogroups A, B, D, and E are similar except for the presence of paratose in serogroup A organisms, this dideoxyhexose is therefore believed to be the immunodominant epitope for MO2. Consistent with the latter contention was the finding that periodate oxidation of the S. paratyphi A lipopolysaccharide did not destroy its antigenicity for MO2. In a slide agglutination test, MO2 was found to react specifically against all 12 clinical isolates of S. paratyphi A but not against 98 isolates of other salmonellae or 74 isolates of other bacteria and Candida albicans.