Pseudomonas aeruginosa, an opportunistic human pathogen, is a major causative agent of mortality and morbidity in immunocompromised individuals and those with cystic fibrosis (CF). In CF patients, the secretion of abnormally high amounts of mucus into the airways contributes to their susceptibility to infection by P. aeruginosa. To identify virulence genes of P. aeruginosa that are important in infection of CF patients, an in vivo selection system (IVET) was used to identify promoters that are specifically inducible by respiratory mucus derived from CF patients. Three genetic loci that are highly inducible by the mucus were identified. One of them is a well-characterized virulence gene (fptA), encoding the receptor for pyochelin, which is a P. aeruginosa iron siderophore. Induction of the fptA gene by mucus is suppressed by the addition of exogenous iron, demonstrating that the mucus is an iron chelator and generates an iron-deficient environment in CF lungs. Therefore, as a part of the host-defence mechanism, the mucus could also be responsible for induction of iron-regulated virulence factors of bacterial pathogens. The second locus, np20, encodes a peptide that shares sequence homology to a number of transcriptional regulators. An identical locus was previously identified to be inducible in vivo during infection of mice and was shown to be important in bacterial virulence in a neutropenic-mouse infection model. The third locus, designated migA (mucus inducible gene), was sequenced and found to encode a 299-amino-acid peptide which is homologous to glycosyltransferases of other bacteria, and is involved in the biosynthesis of lipopolysaccharides or exopolysaccharides. Inducibilities of the np20 and migA genes are not affected by iron and the exact nature of the inducing signals in the mucus is not known. The possible implications of the migA inducibility by respiratory mucus is discussed in relation to the P. aeruginosa infection in CF.