One of the chromosomal segments associated with the virulence of Shigella flexneri and transferred to Escherichia coli K-12 by conjugation has been shown to code for the production of aerobactin and for the synthesis of an iron-regulated 76,000-dalton (76K) outer membrane protein. Analysis of various E. coli K-12-S. flexneri transconjugants showed that the genes involved with the synthesis of aerobactin and with the production of the 76K protein were linked to the mtl region of the S. flexneri chromosome. S. flexneri itself synthesized a 76K protein in its outer membrane under iron restriction as well as traces of 81K and 74K proteins. An examination of four enteroinvasive strains of E. coli showed that each produced aerobactin and a 76K outer membrane protein during iron-restricted growth. The profile of the iron-regulated proteins expressed by the enteroinvasive strains of E. coli was virtually identical to that expressed by the laboratory-constructed E. coli K-12-S. flexneri hybrids under the same growth conditions.