Abstract: A living, attenuated, oral vaccine system is described for the immunization gainst enteric disease. This oral vaccine is a genetic hybrid derivative of an attenuated galactose epimeraseless strain of S. typhi which carries at least one protective antigen other than normal somatic S. typhi antigens. The oral vaccine can provide protection against both typhoid fever and at least one other enteric disease. A bivalent oral vaccine is described wherein the non-typhoid protective antigen is the plasmid-encoded form I antigen of Shigella sonnei. A protective antigen from Shigella sonnei was transferred to a streptomycin resistant mutant of S. typhi strain Ty21a. The transconjugant S. typhi strain expressed both S. typhi and S. sonnei antigens and protected experimental animals against lethal infections with either S. typhi or S. sonnei. This strain is considered to be useful as a vaccine against typhoid fever and bacillary dysentery caused by S. sonnei. The mutated galactose epimeraseless S. typhi strain such as S.