Abstract: Objective: To investigate the possible mechanism of apoptosis of human umbilical vein endothelial cells(HUVECs) induced by Staphylococcus aureus(S.aureus) and its L-forms.Methods: S.aureus and its L-forms were used to infect HUVECs by incubation for different intervals of 2 hours,4 hours,6 hours,8 hours and 10 hours.Apoptotic rates in the treated cells were measured by flow cytometry,and expression of bcl-2 and Fas gene were detected by using immunohistochemical staining.Results: Both S.aureus and its L-forms induced apoptosis in HUVECs.The apoptotic rates of treated HUVECs at different time were significantly higher than that of the control (P<0.05-P<0.01).Morever the apoptotic rates induced by L-forms were obviously lower than that by S.aureus(P<0.01).At the 6 hours after infection,Bcl-2 expression in HUVECs teated with S.aureus and its L-forms was significantly lower,but that of Fas significantly higher than that in the control group.Conclusions: Both S.aureus and its L-forms can induce apoptosis of HUVECs which may be resulted from bcl-2 down-regulation and Fas up-regulation.