Clearly, B. pertussis has evolved very elaborate mechanisms to maintain itself in the human host. Three different proteins (FHA, pertussis toxin and fimbriae) have been implicated in adherence. Furthermore, a number of toxins are produced (pertussis toxin, adenylate cyclase, dermonecrotic toxin, and tracheal cytotoxin) which destroy the clearance mechanisms of the respiratory tract, or suppress the immune response. There is evidence that B. pertussis may survive intracellularly, and the possibility that it is a facultative intracellular parasite should certainly be explored. The availability of a large number of cloned virulence genes, and a system to construct well defined mutants by allelic exchange (Stibbitz et al. 1986) will greatly facilitate the study of Bordetella virulence factors at the molecular level. It opens the possibility to construct avirulent strains, which are still able to colonize and stimulate the local immune response. Such strains may be used as live, oral vaccines, to present (heterologous) antigens to the mucosal immune system of the respiratory tract.