Luminol chemiluminescence in phorbolesteractivated cultured rat liver Kupffer cells was strongly inhibited by the selenoorganic compound ebselen (IC50 = 2 μmol/L). Ebselen (2-phenyl-1,2-benzisoselenazol-3[2H]one) also diminished reduction of ferricytochrome c (IC50 = 10μmol/L), indicating a suppression of superoxide anion formation. Likewise, in lipopolysaccharide-pretreated Kupffer cells, ebselen proved to be a potent inhibitor of the conversion of oxyhemoglobin to methemoglobin (IC50 = 3 μmol/L) as a measure of nitric oxide formation. The sulfurcontaining analog (2-phenyl-1,2-benzisothiazol-3[2H]one) and the ebselen derivative, methylselenobenzanilide, were inactive. These results indicate that ebselen is a potent inhibitor of NADPH oxidase in Kupffer cells, as has been reported for other macrophages and granulocytes. In addition, they suggest a novel characteristic of ebselen, namely very effective inhibition of nitric oxide synthase of macrophages. In line with its inhibitory effects on the release of reactive oxygen species by macrophages, complemented by its antioxidant properties, ebselen was potent in the prevention of reoxygenation injury of Kupffer cells (IC50 ∼ 5 μmol/L). (HEPATOLOGY 1992;15:1112–1116).