Abstract: Interleukin 18 (IL-18) , formerly called IFN-γ-inducing factor, is a novel cytokine that plays an important role in the Th1 response, primarily by its ability to induce IFN-γ production in T cells and natural killer cells. It participates in both innate and acquired immunity. In order to evaluate the possible roles of IL-18 in HBV infection and clarify the host immunity against HBV, we investigated the expression of IL-18 in the peripheral blood mononuclear cells (PBMC) of HBV infectors and normal controls. In 58 HBV infectors (including 15 cases of asymptomatic carriers, 30 cases of chronic hepatitis and 13 cases of fulminant hepatitis) and 10 healthy individuals (as normal controls) , IL-18 expression levels in PBMC were quantitatively analyzed with flow cytometric immunological method. The PBMC were separated routinely and stimulated with LPS (SIGMA) and Monensin (SIGMA) for 6 hours. Then the cells were harvested and fixed by PBS/4% parafomaldehyde. The treated cells were stored in liqud nitrogen for detection later. After immunogic stain, the expression of IL-18 in PBMC were examined by flow cytometry. (1) There were significant differences between each type of infection. The level of IL-18 in fulminant hepatitis was highest. Asymptomatic carriers expressed fewer IL-18 than normal controls did (P<0 01) . There was no significant difference between chronic hepatitis and normal controls (P=0.25) . (2) In the group of chronic hepatitis, the results suggested that the expression of IL-18 correlated with the degree of liver inflammation. (3) The IL-18 expression level in chronic hepatitis significantly correlated with serum ALT positively (r=0.63, P<0 01) . Conclusion:IL-18 may take part in host immunity against HBV infection and relate to the severity of liver inflammation.