Neutrophils accumulated in the lung are thought to play a key role in the pathogenesis of adult respiratory distress syndrome and interstitial pneumonia following bone-marrow transplantation. The effects of gabexate mesilate on several aspects of human neutrophil function have been investigated. Gabexate mesilate significantly decreased both the generation of reactive oxygen species (O2-, H2O2, OH.) by neutrophils and neutrophil chemotaxis. In contrast, the drug did not affect the levels of reactive oxygen species generated by a cell-free reactive-oxygen-species generating system. Intracellular calcium concentrations in neutrophils stimulated by f-Met-Leu-Phe were decreased in the presence of gabexate mesilate. These data suggest that the reduction in reactive-oxygen-species production and neutrophil chemotaxis by gabexate mesilate may contribute to the effectiveness of the drug in adult respiratory distress syndrome and interstitial pneumonia after bone-marrow transplantation. The suppression of the increase in intracellular calcium concentration may at least be responsible for the inhibition of these neutrophil functions by gabexate mesilate.