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Representative Shigella flexneri strains were studied to determine whether plasmids are involved in their virulence. All invasive S. flexneri strains, irrespective of serotype, were found to harbor a large plasmid of approximately 140 megadaltons in size, although some strains carried additional plasmid species. Spontaneous variants of strains of serotypes 1, 2, and 5 had lost this 140-megadalton plasmid and had concomitantly become avirulent, i.e., could neither invade HeLa cell monolayers nor produced keratoconjunctivitis in guinea pigs. To monitor plasmid transfer, the 140-megadalton plasmid of strain M90T (serotype 5) was tagged with the kanamycin resistance transposon Tn5. This tagged plasmid, pWR110, was not self-transmissible, but was mobilized by one of several different conjugative plasmids into avirulent derivatives of the heterologous serotypes 1 and 2 which had lost the comparable large plasmid. Transconjugants of both serotypes which had received pWR110 regained virulence. These data directly demonstrate that this large S. flexneri plasmid encodes or regulates some function(s) required for epithelial cell penetration.