The tracheobronchial secretions from patients with cystic fibrosis often contain high amounts of free proteases. To evaluate whether human leucocyte elastase (HLE) can favour the persistence of bacterial airways infection, we exposed the frog palate mucosa to HLE and then to radiolabelled Pseudomonas aeruginosa and followed the sequence of events by scanning electronmicroscopy. In response to HLE there was a marked outpouring of mucus and a desquamation of the epithelium. P. aeruginosa was shown to adhere to recently secreted granules of mucus and to the exposed submucosal underlying connective tissues. For the eight different bacterial strains studied, a significative adherence to HLE-injured mucosa was observed only in strains that possessed internal haemagglutinating activity. Neither the presence of fimbriae, nor of the mucoid exopolysaccharide, nor of the bacterial surface haemagglutinating activity could be related to adherence of P. aeruginosa to the injured mucosa. These results support the hypothesis that HLE enhances bacterial infection of the respiratory mucosa both by inducing mucus hypersecretion and by exposing receptors to the microbial adhesins. It is also suggested that P. aeruginosa internal lectins may be implicated in adherence to host tissues.