There is disclosed a novel immunogen which can be used for immunological protection against acute diarrheal disease caused by enterotoxigenic strains of Escherichia coli. The novel immunogen is provided by cross-linking the E. coli heat-stable enterotoxin with the E. coli heat-labile enterotoxin (either the complete holotoxin or just the B subunit of this toxin) by a conjugation process in the presence of a conjugating agent, typically the water soluble carbodiimide, 1-ethyl-3-(3-diamethylaminopropyl) carbodiimide. This results in a unique new molecule in that the toxic properties of each individual toxin are greatly reduced, the heat-labile toxin (or its B subunit) retains its antigenicity, and the heat-stable toxin acquires immunogenicity as a function of the reaction. Immunization with the cross-linked immunogen provides immunological protection in mammals against water secretion induced by strains of E. coli which produce the heat-labile or heat-stable enterotoxins, either singly or together.