Abstract: A recombinant strain of B. anthracis is characterized in that it can induce the production of protective antibodies against virulent strains of B. anthracis in a human or animal host, and characterized also by the mutation of the pX01 plasmid of at least one given gene coding for a protein which causes a toxic effect of B. anthracis, wherein said mutation leads to the deletion of all or part of said gene which codes for the protein causing the toxic effect, and to the insertion of a DNA cassette at said gene's deletion site in pX01, whereby the strain thereby modified may be selected and a back mutation of the recombinant strain may be prevented, and wherein the gene thereby mutated is thereafter either unable to produce the protein causing the toxic effect for which it codes, or able to code for a truncated protein which has lost its toxic properties. The use of such a strain in immunogenic compositions is also described.