During acute lung inflammation, the airspaces are invaded by circulating neutrophils. These may then injure tissues through the release of elastase. Different natural specific inhibitors such as 1-PI, SLPI and elafin, are nonetheless able to counteract the enzymatic activity of elastase. The present study was undertaken to assess the role of these different inhibitors in the intrinsic anti-elastase barrier during LPS-induced lung inflammation in mice. Upon intranasal administration of LPS to mice, the anti-elastase activity recovered from bronchoalveolar lavage fluids (BALF) increases progressively up to 48 h (7-fold) and returns to the basal level within 72 h. By contrast, when the same experiments were performed with neutropenic mice (pretreatment with an anti-granulocyte antibody, or vinblastine), the increase was totally absent. Ultrafiltration of BALF through 100 kDa cut-off membranes, shows that the activity remains in the retentate thus ruling out a role for native 1-PI, SLPI and elafin. Gel filtration and fraction analysis show that the material eluted with a Mr of 600 kDa. Agarose gel electrophoresis and ethidium bromide staining reveal that the activity corresponds to the presence a large amount of DNA. Interestingly, DNAse treatment of the active fraction suppresses the anti-elastase activity. Analysis of BALF from ALI patients shows the presence of DNA with anti-elastase activity. We therefore concluded that during acute lung inflammation the recruitment of neutrophils in the airspaces accounts for the increased presence of DNA which in turn contributes to the anti-elastase barrier.