Triggered human neutrophils were able to maintain released elastase in an active form in the presence of purified alpha-1-proteinase inhibitor (alpha-1-PI), serum or bronchoalveolar lavage fluid (BAL). The accumulation of free elastase activity was associated with a decrease in the ability of the alpha-1-PI to inhibit porcine pancreatic elastase, an increase in proteinase activity associated with alpha-2-macroglobulin, and the oxidation of alpha-1-PI to a molecule containing four methionine sulfoxide residues. Neutrophils used both hypochlorous acid and long-lived N-chloroamines to oxidize the alpha-1-PI, but hypochlorous acid was preferentially used for suppressing the activity of the antiproteinase over short distances whereas the N-chloroamines were effective even when the phagocytes and alpha-1-PI were physically separated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified alpha-1-PI, serum, or BAL that had been incubated with triggered neutrophils revealed that the released neutrophil elastase was not complexed with the antiproteinase and that a portion of the alpha-1-PI had undergone proteolysis. These data suggest that the presence of free neutrophil elastase as well as inactive, oxidized, and proteolyzed alpha-1-PI in fluids recovered from inflammatory sites in vivo could be directly mediated by triggered neutrophils alone.