In cystic fibrosis, impaired mucociliary clearance leads to chronic endobronchial bacterial infection, mostly caused by Pseudomonas aeruginosa. In the early stage of infection, the pathogen produces several extracellular protein toxins which may contribute to its multifactorial virulence before specific antibodies are produced. P. aeruginosa successfully resists phagocytosis by neutrophils, which dominate the local inflammatory response, by switching from a nonmucoid variant to a mucoid phenotype. Chronic infection and inflammation are characterized by neutrophil-released proteinases which may provide favourable growth conditions for the bacterial opportunist. Aerosol therapy with serine proteinase inhibitors is being investigated in cystic fibrosis.