Neutrophil accumulation in the lung may contribute to tissue injury as observed in inflammatory diseases. Both oxidative and non-oxidative mechanisms are involved in neutrophil-mediated tissue injury. Non-oxidative mechanisms include the release of neutrophil granule proteins such as the serine proteinases elastase and cathepsin G, and the non-enzymatic defensins. Because stimulated neutrophils are thought to release their products simultaneously, we investigated possible interactions between purified defensins and serine proteinases with respect to induction of cellular injury and their ability to induce interleukin-8 (IL-8) synthesis in cells of the lung epithelial cell line A549. Whereas defensins induced cell lysis, elastase and cathepsin G induced detachment of A549 cells. Co-incubation of elastase and cathepsin G revealed an additive effect on detachment, whereas defensins inhibited serine proteinase-induced detachment. Vice versa, both serine proteinases reduced defensin-induced cell lysis. Furthermore, elastase and cathepsin G prevented defensin-induced IL-8 synthesis. In contrast, no inhibitory interaction between cathepsin G and defensins was observed with respect to their antibacterial activity. The results from this study indicate that, at sites of inflammation, neutrophil-mediated injury might be regulated by interactions between released defensins and serine proteinases.