Abstract: Objective To investigate the effects and mechanisms of benzo(a)pyrene(BaP) on cell-cycle of cultured human bronchial epithelial(16HEB) cells.Methods Cell viability was assessed by the Cell Counting Kit-8(CCK-8) kit assay. Flow cytometry(FACS) analyses were performed on 16HBE cells 24 hours after 0,5,10,20,40,and 80 μmol/L BaP treatment,and we detected the expression levels of ataxia telangiectasia-mutated gene(ATM) and p53 using real-time reverse transcription PCR(RT-PCR) and western blot.Results BaP induced significant concentration-dependent inhibition in the cell proliferation and BaP enhanced S-phase arrest or delayed S-phase transition in 16HBE cells.RT-PCR and western blot analyses showed that BaP induced concentration-dependent increase in expression of ATM and p53. Conclusion BaP could prolong S-phase in 16 HBE cells and ATM and p53 play important roles in the response to the cell-cycle change induced by BaP.