Cystic fibrosis (CF) is the most common genetic disease of Caucasians with an incidence of 1 in 2500 live births and a carriage rate of 1 in 20 individuals. CF patients continue to suåer from recurrent and chronic respiratory tract infections and most of their morbidity and mortality is due to such infections throughout their life (Høiby, 1991). These infections are usually dominated by Gram-negative organisms, especially by the pseudomonads including in particular, Pseudomonas aeruginosa, as well as members of the Burkholderia cepacia complex of organisms.