Abstract: ObjectiveTo research the role of Vi antigen of S.typhi as a virulence factor in pathogenic mechanisms of S.typhi.MethodsS.typhi strains with positive and negative Vi antigen were used.The role of Vi antigen was examined in resistance of osmotic perssure(NaCl)and preoxide(H₂O₂)kill,as well as against internalizing and intracellular killing by mononuclear phagocytes.ResultsThe survival of Vi negative S.typhi strains was decreased significantly in the presence of high concentration of NaCl and of low concentration of H₂O₂;Vi negative S.typhi strains were internalized and killed dramat-ically by mononuclear phagocytes compared with the control group.ConclusionThese results indicated that Vi antigen of S.typhi strains as a virulence factor possessed a fatal role in pathogenic mechanisms of S.typhi by resistance to host defense.