AIM: To investigate the effects and mechanisms of melatonin on immunological liver injury in mice.METHODS: A model of liver injury was induced by tail veininjection of Bacillus Calmette Guerin (BCG) and lipopolysaccharide(LPS) in mice. Kupffer cells and hepatocytes were isolatedand cultured according to a modified two-step collagenaseperfusion technique. Levels of alanine aminotransferase(ALT), aspartate aminotransferase (AST) and nitric oxide(NO), content of malondiadehyde (MDA), activity of superoxidedismutase (SOD), were measured by biochemical methods.Tumor necrosis factor-α (TNF-α) activity was determinedby RIA. Interleukin (IL)-1 activity was measured by thymocyte proliferation bioassay. Hepatic tissue sections were stained with hematoxylin and eosin and examined under a lightmicroscope.RESULTS: Immunological liver injury induced by BCG+LPSwas successfully duplicated. Serum transaminase (ALT,AST) activities were significantly decreased by melatonin(0.25, 1.0, 4.0 mg/kg bm). Meanwhile, MDA content was decreased and SOD in liver homogenates was upregulated.Furthermore, pro-inflammatory mediators (TNF-α, IL-1, NO)in serum and liver homogenates were significantly reduced by melatonin. Histological examination demonstrated that melatonin could attenuate the area and extent of necrosis,reduce the immigration of inflammatory cells. In in vitro experiment, TNF-α was inhibited at the concentrations of10-8-10-6 mol/L of melatonin, while IL-1 production of Kupffer cells induced by LPS (5 μg/mL) was decreased only at theconcentration of 10-6 mol/L of melatonin, but no effect onNO production was observed. Immunological liver injury model in vitro was established by incubating hepatocyteswith BCG- and LPS-induced Kupffer cells. Activities of ALT,TNF-α, IL-1, and MDA in supatant were significantlyincreased. Melatonin had little effect on the level of ALT,but reduced the content of TNF-α and MDA at concentrationsof 10-7-10-5 mol/L and decreased the content of IL-1 atconcentrations of 10-6-10-5 mol/L.CONCLUSION: Melatonin could significantly protect liverinjury in mice, which was related to free radical scavenging,increased SOD activity and pro-inflammatory mediators.