The effects of glycyrrhizin isolated from licorice root were investigated on acute hepatitis induced by lipopolysaccharide (LPS) and d-galactosamine in mice. Serum alanine aminotransferase (ALT) activity was markedly increased 6 h to 8 h after administration of LPS/d-galactosamine. Levels in serum of cytokines such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-10 and IL-12 reached a maximum by 2 h, whereas levels of IL-18, as well as of ALT, were maximal at 8 h. Glycyrrhizin (ED(50): 14.3 mg/kg) inhibited the increase in ALT levels when it was given to mice at 30 min before administration of LPS/d-galactosamine. Inflammatory responses, including infiltration of neutrophils and macrophages in the liver injury, were modulated by glycyrrhizin. Increases in ALT levels were reduced by an administration of glycyrrhizin at 10 min and 60 min but not 3 h, even after LPS/d-galactosamine treatment. However, glycyrrhizin had no effect on the production of TNF-alpha, IL-6, IL-10 and IL-12, whereas it significantly inhibited IL-18 production. Exogenous IL-18 further increased the elevation in ALT levels in mice treated with LPS/d-galactosamine. Glycyrrhizin completely suppressed the effect of IL-18 of increasing ALT levels. IL-18 was detected by immunohistochemistry in inflammatory cells such neutrophils and macrophages in liver injury. Glycyrrhizin reduced the responsiveness of cells to IL-18 in the liver injury. These results suggest that glycyrrhizin inhibits the LPS/d-galactosamine-induced liver injury through preventing inflammatory responses and IL-18 production. Furthermore, it seems that glycyrrhizin prevents IL-18-mediated inflammation in liver injury.