Abstract: Objective To explore the effect of benzo(a)pyrene(Bap)on cell cycle of synchronized human embryonic lung fibrob last(HELF).Methods Treatments of 0.5% serums tarvation and 10% serumre stimulation were used to obtain cells synchronisation in G0/G1,S and G2/Ms tage.H ELF cells synchronized in specific phase after serumre-stimulation were exposed to dime thylsulfoxide,2,10 and 50μmol/LB aP for 2 hours.Cell cycle distribution and cell cycle regulation proteins such as cyclin D,cyc lin E,cyc lin A,cyc linB,P53,P21 and P16 were detecred 12hr after B aP exposure with FCM and Western Blotting methods,respectively.Results Serumstarvation and restimulation induced cells arrest in different phase and cell cycle phase change obviously 24 hr after serumre-stimulation.Cyclins expression in HEL F cells deceased from G0 phase in a time-dependent manner.BaP induced a decrease of HEL F cells in S stage.HEL F treated with Bap were a rrested in G1s tage and the expression of cyclin D decreased distinctly after serumre-stimulation of 102.2 hr.HEL F with Bap treatment 16-18 hr after serum restimulation exhibited inceases of G2 stage and P53,P21 expressions.The ratio of G0/G1 and the expressions of P53 and P21 inc reased in HELF treated with Bap 22-24 hr after the serum restimulation.Conclusion BaP activates both G1 and G2 checkpoints of HELF synchronized in specific phase and induces cell cycle a rrest.