[en] The radiosensitivity of DNA molecules in testicular cells from adult male mice in situ was 0.22 single-strand breaks per 10¹² daltons of DNA per rad. This was significantly lower than 0.6-0.7 for liver and thymus DNA, and was most likely attributable to a hypoxic condition of testicular cells in situ. Although a majority of the single-strand scissions was quickly rejoined, a significant extent of breaks remained without further rejoining. The presence of remaining breaks was demonstrated not only after 3-6 h of post-irradiation time, but also after irradiation at low dose rates. The remaining breaks might be a slow and/or non-reparable DNA damage