Cross-agglutination and absorption studies indicated that two serological reference strains of Listeria grayi and Listeria murrayi differed in at least one O factor. The serological analysis revealed that the unshared O factors are not present in other Listeria serovars. These results are not in accordance with the antigenic structure previously reported for L. grayi and L. murrayi, then a new O antigen formulation is proposed for these species: L. grayi (III), XII, XIV and XVI; L. murrayi (III), XII, XIV and XVII.