Whooping cough, an acute respiratory disease affecting over sixty million infants, can be prevented by vaccination. The vaccine currently used, composed of killed bacterial cells, however, has been associated with many side effects. An improved vaccine against the disease should contain pertussis toxin (PT), a major virulent factor of Bordetella pertussis (B. pertussis). In order to be included in the vaccine, PT needs to be detoxified and the chemical methods used so far are not completely satisfactory, since they give a product with reduced immunogenicity and possible residual toxicity. To avoid this problem, we have used recombinant DNA technologies to clone the PT gene, express it in bacteria, map the B and T cell epitopes of the molecule and identify the amino acids that are important for the enzymatic activity and toxicity. Based on this information, the gene coding for PT was mutated to produce an inactive protein. This genetically modified PT was non toxic, highly immunogenic and able to protect mice from intracerebral challenge with virulent B. pertussis. The mutant was included as a main component of an acellular pertussis vaccine which has been shown in numerous clinical trials to be more safe and immunogenic than the old cellular vaccine.