Abstract: Objective To observe the effect of actinomycin D(ACTD)and conditioned medium obtained from ACTD-exposed V79 cells on cell morphology and ultrastructure to determine whether ACTD could induce bystander effect in V79 cells. Methods V79 cells were administrated with ACTD for 1 hr at different dose(0,0.25,0.5,1.0,2.0,4.0,and 8.0 mg/L).Different period conditioned medium(CM)was collected at 4,8,12,and 24 hr after ACTD treatment to culture bystander cells for 24 hr to observe the bystander effect.Cell morphology and ultrastructural changes were observed under phase contrast optical microscope and transmission electron microscopy,respectively. Results The numbers of V79 cells reduced and those of abnormal cells increased in ACTD-treated groups in a dose-dependent manner(P<0.05).Bystander cells were much less and the cells turned round and many cells got rid of wall.The bystander cells damage alleviated with the time but aggravated in 24 hr CM-treated bystander cells.Under transmission electron microscopy,the cells were in normal shape with normal organelles and nuclear.In 4 mg/L ACTD-exposed cells,shrinked and apoptotic bodies formed. Amongst the bystander cells treated with 4 hr CM,apoptosis phenomenon was the most obvious and apoptosis reduced with the time till a rebound in 24 hr CM.Death type of the bystander cells was mainly apoptotic.Survival rates of the bystander cells in 4,8,12,and 24 hr CM groups were 59.5±3.4%,69.2±4.5%,88.8±5.2%,and 61.3±6.8%,respectively, which all were lower than that of the control group(100.0±0.1%)and higher than that of 4 mg/L ACTD group(50.0± 6.5%). Conclusion ACTD could induce apoptosis of bystander V79 cells.The 4 hr CM treatment induces the strongest bystander effect.