Abstract: Objective: To explore the effect of StaphylococcuS.aureus(S.aureus) L-forms on apoptosis of murine thymocytes.Methods: Murine thymocytes were co-cultured with S.aureus,S.aureus L-forms or staphylococcal enterotoxin B(SEB) in vitro for different times(2 hrs,4 hrs,6 hrs,8 hrs,or 10 hrs) before examination.The apoptotic rates of murine thymocytes were measured by flowcytometry technique and using Annexin V-FITC/PI double staining kit.Results: The apoptotic rates of thymocytes in group of SEB treated for 2 hrs,in group of S.aureus infected for 4 hrs,and in group of S.aureus L-forms infected for 6 hrs were obviously increased compared to that of the control(all groups showed P<0.01).The longer the time of incubation,the higher rates of thymocyte apoptosis.Conclusions: Both S.aureus L-forms and S.aureus can induce the apoptosis of murine thymocytes.Except for SEB,other factors might be involved in the apoptosis of thymocytes induced by S.aureus and S.aureus L-forms that producing SEB.