[en] In bronchoalveolar lavage (BAL) fluid from ventilated patients, cytotoxic oxidant activity is correlated with neutrophil activation. The aim of the present study was to investigate the hypothesis that BAL fluid induces activation of the transcription nuclear factor-kappaB (NF-kappaB) in human alveolar cells, in correlation with inflammatory mediators. We measured endotoxin, inflammatory cytokines [Interleukin-1beta (IL-1beta), IL-8], nitrated proteins and the activity of myeloperoxidase (MPO) in BAL fluid from ventilated patients developing bronchopneumonia (n = 19 samples) or with acute respiratory distress syndrome (ARDS) (n = 14), and from ARDS/infection-free patients (n = 11). We also exposed alveolar cells to the BAL fluid or to human MPO, H2O2 or HOCl, and tested nuclear extracts for the activation of NF-kappaB. IL-1beta, IL-8, nitrated protein, MPO and endotoxin levels were significantly higher in BAL fluid from patients with bronchopneumonia than in that from the ARDS and ARDS/infection-free groups. A correlation was observed between IL-8 and MPO values (r = 0.82). The level of NF-kappaB activity induced by the BAL fluid was correlated with levels of IL-1beta (P < 0.001), IL-8 (P < 0.005) and MPO (P < 0.002), and with the neutrophil count (P < 0.002), and was higher for BAL fluid from the bronchopneumonia group. NF-kappaB activation by MPO was also demonstrated. The activation of NF-kappaB by BAL fluid, especially that from bronchopneumonia patients, suggests that a similar phenomenon may occur in vivo, leading to potential amplification of the inflammatory reaction.