Epithelial cell detachment from underlying basement membrane is a feature of diseases of many organs. In the lungs it is seen in disorders as diverse as bronchiectasis, allograft rejection, and asthma. The potential for different leukocytes to induce this change is not clear. In asthma both eosinophils and neutrophils are found in affected tissues, but the capacity of each of these types of cells to induce detachment of native epithelial cells from basement membrane requires clarification. Although eosinophils damage rather than detach human epithelial cells, the effects of neutrophils on epithelial cells naturally attached to basement membrane have not previously been described. Using the human amnion in vitro model, we tested the hypothesis that neutrophils have the capacity to detach intact human epithelial cells from basement membrane. The data indicate that increasing concentrations of neutrophils are able to detach epithelial cells from their underlying basement membrane. Detachment was increased when the neutrophils were activated in situ with tetradecanoyl phorbol acetate and after longer incubation periods. Platelet activating factor and opsonized zymosan showed similar boosting effects, whereas activated complement and formyl-methyl-leucyl-phenylalanine did not. Physical contact of the neutrophils with the epithelial cells was required to induce detachment. Detachment could be inhibited by glutathione and by soybean trypsin inhibitor, an inhibition pattern similar to cathepsin G and trypsin, but not collagenase, in this system. We conclude that neutrophils are capable of detaching human epithelial cells from basement membrane, which in part involves the release of chymotrypsin-like serine proteases, probably in conjunction with oxidants, and that this detachment can be inhibited. (J ALLERGY CLIN IMMUNOL 1995;95:597-606.)