Abstract: OBJECTIVE: To observe the effect of actinomycin D (ACTD) and conditioned medium on cell viability and chromosome aberration in hamster fibroblast V79 cells to determine whether actinomycin D could induce bystander effect. METHODS：V79 cells were treated with ACTD for 1 h at different doses (0，0.25，0.5，1.0，2.0， 4.0 and 8.0 mg/L). The 4 mg/L ACTD- conditioned medium(CM) was collected at 4，8，12 and 24 h after treatment for 1 h to culture bystander cells to observe the bystander effect. Cell viability was evaluated by MTT assay；chromosome damage was detected using the conventional analysis of chromosomal aberrations. RESULTS：Cell viability of ACTD-treated V79 cells was reduced in a dose-dependent manner (P<0.05)，was close to 50 percent and stable when ACTD was at 4 mg/L for 1 h. Cell viability of CM-treated bystander cells increased significantly in a time-dependent manner (P<0.05) till a rebound in 24 h CM (P<0.05). The bystander effect in the 4 h CM was the strongest. Chromosome aberration rate in ACTD-treated cells increased significantly in a ACTD dose-dependent manner (P<0.05). Chromosome aberration rate in CM-treated bystander cells decreased significantly in a time-dependent manner (P<0.05) till a rebound in 24 h CM，with the 4 h CM being the highest. Type of chromosome aberration in bystander cells was mainly breakages along with ring chromosome，fragmented and dicentric chromosome similar to exposed cells. CONCLUSION：ACTD could induce bystander effect in V79 cells. Chromosome aberration type consisted of mainly breakages in both ACTD-treated cells and CM-treated bystander cells.

Key words: actinomycin D, conditioned medium, bystander effect, choromosome aberration