An immunogenic detoxified protein is provided which comprises the amino
acid sequence of subunit A of an E. coli heat labile toxin (LT-A) or a
fragment thereof in which at least amino acid Ala-72 of the A subunits
mutated, preferably by substitution with Arg. The toxoid is useful as
vaccine against an enterotoxigenic strain of E. coli and is produced by
recombinant DNA means by site-directed mutagenesis. It is also an
effective adjuvant.