DNA content and cell cycle of irradiated HeLa S3 cells in the exponential phase was measured by image analysis and flow cytometric analysis. The HeLa S3 cell phases had undergone remarkable changes after irradiation. There was a dose dependent radiation effect. As the radiation dose increased, the number of G0 + G1 and S phase cells were reduced and G2 + M phase delay occurred. The number of G2 + M phase in HeLa S3 cells after 6-8 Gy was increased from 22% to 52%-61%. Meanwhile, the DNA content of HeLa S3 cells was increased from 6.72 to 9.08-10.0 Au, the results of these two assays were compatible. Most of the delay of G2 + M phase appeared 24 hours after irradiation. The concentration of HeLa S3 cells used varied from 32 x 10(4)/ml to 147 x 10(4)/ml in the experiments. No relationship was observed in the changes of phase and cell concentrations.