We investigated the mechanism by which the synthetic protease inhibitor gabexate mesilate inhibits the production of the superoxide anion by human neutrophils. We found that gabexate mesilate suppressed SOD-inhibitable cytochrome c reduction in a dose-dependent manner in intact neutrophils activated with phorbol ester. Gabexate mesilate slightly scavenged the superoxide anion in the pyrogallol assay. The reagent also inhibited superoxide anion production in a dose-dependent manner in a cell-free oxidase-activating system. Translocation of the cytosolic respiratory burst oxidase components, the 47- and 65-kDa proteins, to membranes was suppressed by the reagent in intact cells stimulated with phorbol ester. Gabexate mesilate also reduced arachidonic acid-induced translocation of the components to the membrane fraction in the cell-free system. These results demonstrate that gabexate mesilate suppresses superoxide anion production by reducing the translocation of the 47- and 65-kDa proteins to the plasma membrane.