Neutrophil proteinase-mediated lung tissue destruction is prevented by inhibitors, including elafin and its precursor, trappin. We wanted to estab-lish whether neutrophil-derived oxidants might impair the inhibitory func-tion of these molecules. Myeloperoxidase⁄H2O2 and N-chlorosuccinimide oxidation of the inhibitors was checked by mass spectrometry and enzy-matic methods.