The group factor 3,4 somatic antigen was transferred by recombination from a donor strain of Shigella flexneri type 2a to the Escherichia coli O15 rabbit pathogen strain RDEC-1. A hybrid clone (7482-1-1) that expressed only the S. flexneri 3,4 somatic antigen and a second clone (7482-1-7) from the same mating mixture that expressed only the O15 somatic antigen were compared for virulence in rabbits. The 7482-1-1 strain produced diarrhea in 10% of rabbits versus 84% for the 7482-1-7 strain (P less than .001). In fluorescent antibody-stained, frozen, sectioned tissues, fewer 7482-1-1 bacteria adhered to ileum (P = .013) and cecum (P = .044). Light and electron microscopic studies demonstrated that the recombinant strain, which adhered to membranous (M) cells to the same degree as the 7482-1-7 strain, was found beneath the epithelium, where its presence resulted in a marked acute inflammatory response. Both the 7482-1-1 and 7482-1-7 strains exhibited characteristic close adherence to M cells and absorptive ileal and cecal mucosa.