Invasive Escherichia coli is a "Shigella-like" microorganism which causes a dysenteric syndrome through invasion of the human colonic epithelium. Representative strains of different serotypes were studied in order to determine whether plasmids are involved in their virulence. All invasive E. coli strains, irrespective of serotype, were found to harbour a large plasmid of approximately 140 Mdal. Spontaneous variants of serotypes O143 and O124 had lost this plasmid and had become avirulent, i.e. could neither penetrate into HeLa cells nor produce a keratoconjunctivitis in guinea-pigs. pWR110, a Tn5-labelled virulence plasmid of Shigella flexneri, was transferred into these avirulent variants, thus restoring their virulence and demonstrating that S. flexneri and invasive E. coli share a common extrachromosomal control of their ability to penetrate into cells.