The DNA double-strand breaks (DSBs) are considered to be the most relevant lesions for the deleterious effects of ionizing radiation exposure. The discovery that the induction of DSBs is rapidly followed by the phosphorylation of H2AX histone at Ser-139, favoring repair protein recruitment or access, opens the possibility for a wide range of research. This phosphorylated histone, named γ-H2AX, has been shown to form foci in interphase nuclei as well as megabase chromatin domains surrounding the DNA lesion on chromosomes. Using detection of γ-H2AX on germ cell mitotic chromosomes 2 h after γ-irradiation, we studied radiation-induced DSBs during the G₂/M phase of the cell cycle. We show that 1) non-irradiated neonatal germ cells express γ-H2AX with variable patterns at metaphase, 2) γ-irradiation induces foci whose number increases in a dose-dependent manner, 3) some foci correspond to visible chromatid breaks or exchanges, 4) sticky chromosomes characterizing cell radiation exposure during mitosis are a consequence of DSBs, and 5) γ-H2AX remains localized at the sites of the lesions even after end-joining has taken place. This suggests that completion of DSB repair does not necessarily imply disappearance of γ-H2AX.