The present report described the production and characterization of a monoclonal antibody, TMY1, specific for an O-antigen of Salmonella bacilli. The following results were obtained: The slide agglutination test against strains of Salmonella serovars indicated the responsiveness of TMY1. TMY1 was reactive only to strains that also possessed O5-antigen in group O4. The agglutinating ability of TMY1 was absorbed completely with bacilli possessing O5-antigen in group O4. When treated with 1N HCl, bacilli possessing O5-antigen in group O4 showed no agglutinability in the presence of TMY1. The data indicates that TMY1 was specific for O5-antigen. Strains of Salmonella group O4 were classified by the agglutination test with TMY1 into two subgroups, O5-antigen positive and negative, suggesting the usefulness of TMY1 as an epidemiologic tool for the serotyping of Salmonella.