All of the Salmonella dublin organisms isolated from cattle in Japan (20 strains) and in Europe (2 strains) harbored one plasmid of approximately 50 megadaltons (Mdal). The biochemical reactions of a derivative strain (5230P-) cured of the 50-Mdal plasmid were those of its parental strain 5230P+. However, the strain 5230P- proved to be 100- to 1,000-fold less virulent for mice compared with that of the strain 5230P+, suggesting that the virulent phenotypes of S. dublin may be mediated by the 50-Mdal plasmid.