Exponentially growing human tumor cell lines were γ -irradiated with a dose of 3Gy. The DNA content and percentage of each cell-cycle phase were obtained with flow cytometry. Results indicated that for SMMC-7721

hepG2 and HO-8910 cells

G2/M accumulation was prominently induced by irradiation and the maximal accumulation appeared at 12 h after treatment

and that S phase just displayed temporal accumulation

i.e. S phase cells accumulated at 6 h after irradiation

and thereafter decreased close to or below the level of un-irradiated cells

and that for Hela cells both G2/M and S phase accumulation were pronouncedly triggered by irradiation. It is suggested that there remain differences in activation and maintenance of cell cycle checkpoints after irradiation among these four tumor cell lines.