Depelestat is a small-protein neutrophil elastase (NE) inhibitor. The elastase at the neutrophil surface was fully inhibited by depelestat and formed soluble complexes. The elastase in cystic fibrosis sputum supernatants was inhibited by stoichiometric amounts of depelestat, allowing titration of the protease. But the percentage of inhibition in whole sputum homogenates varied from 50 to 100%. Depelestat could reduce neutrophil trans-epithelial migration and reduce activity released from neutrophils and NE-induced cytokine expression in airway epithelial cells. NE inhibition could be useful in managing neutrophilic airway inflammation in cystic fibrosis.