A monoclonal antibody, SBY1 (IgM, kappa), against the Salmonella O-antigen was generated by using the myeloma cell line Sp2/O-Ag14 as a fusion partner with spleen cells from BALB/c mice immunized with S. senftenberg 963 K. SBY1 was characterized by the slide agglutination and absorption test. SBY1 was believed to show the specificity to O1-, O3- or O19-antigens of Salmonella because S. Senftenberg 963 K (O1, 3, 19) was used as the antigen for immunization. The slide agglutination test with the Salmonella serovars indicated the responsiveness of SBY1. SBY1 was reactive only with strains that possessed O19-antigen. The agglutinating ability of SBY1 was absorbed completely with bacilli possessing O19-antigen. These finding indicates that SBY1 is specific for O19-antigen. Polyclonal factor sera for he serotyping of the O3, 10 group of Salmonella cross-reacted with Salmonella group O1, 3, 19 in the slide agglutination test. In contrast, SBY1 did not cross-react with serovars from several other Salmonella groups. These data suggest the usefulness of SBY1 as a serodiagnostic tool for serotyping of Salmonella.