Cloning and sequencing of the Eco RI fragment of B. pertussis chromosomal DNA with 4696 base pairs, containing the genes which code for the five subunits of the pertussis toxin. A hybrid plasmid containing the DNA fragment or its further fragments and a micro-organism transformed by the hybrid plasmid and capable of expressing the cloned DNA fragment or further fragments thereof by synthesis of the pertussis toxin or one or more subunits of the pertussis toxin. The pertussis toxin or one or more subunits of the pertussis toxin so obtained are useful for the preparation of vaccines and diagnostic kits.