Exponentially growing human erythroleukemia K562 cells were synchronized by centrifugal elutriation prior to and after Co60 gamma-irradiation (4 Gy). Forward scatter flow cytometry used for size analysis revealed the increase of an early apoptotic cell population ranging from lower (0.05 C-value) to higher DNA content (approximately 1 C) as the cells progressed through the S phase. The increase in cellular DNA content expressed in C-values correlated with apoptotic chromatin changes manifested as many small apoptotic bodies in early S phase and larger but less numerous disintegrated apoptotic bodies in late S phase. Most significant changes after exposure to gamma-irradiation took place in early S phase resulting in an increase of nuclear size by more than 50%. Cell fractions containing irradiated cells showed enhanced growth arrest at 2.4 C-value, which was accompanied by apoptosis. Apoptotic cell cycle arrest near to the G1/G0 checkpoint and apoptotic changes indicate that the radiation resistance of K562 cells is related to the bypass of the early stage of the p53 apoptotic pathway. Apoptotic changes in chromatin structure induced by gamma-irradiation indicate that these injury-specific changes can be identified and distinguished from chromatin changes induced by UV radiation or heavy metals.