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, numbered relative to SEQ ID NO:1, 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.