Sertoli cells isolated from the adult mouse and human testis resume proliferation in culture. After 20 days of culture in Dulbecco modified Eagle medium/Ham F12 (DMEM/F12) medium containing 5%% fetal calf serum, about 36%% of the mouse Sertoli cells, identified by their immunohistochemical staining for the Sertoli cell marker vimentin, incorporated bromodeoxyuridine (BrdU). The renewed proliferation was associated with a 70%% decrease in expression of the cell cycle inhibitor CDKN1B (P27^kip1) and a 2-fold increase in the levels of the proliferation inducer ID2. In vivo, the balance between cell cycle inhibitors and inducers probably is such that the cells remain quiescent, whereas in culture the balance is disturbed such that Sertoli cells start to proliferate again. The renewed proliferative activity of Sertoli cells in culture was further confirmed by double staining for BrdU and the Sertoli cell marker clusterin (CLU), showing about 25%% of the CLU-positive Sertoli cells to be also positive for BrdU after 13 days of culture. Radiobiologically, Sertoli cells are also different from other quiescent somatic cells in the testis because they express several DNA repair proteins (XRCC1, PARP1, and others). Indeed, a comet assay on irradiated Sertoli cells revealed a 70%% reduction in tail length and tail moment at 20 h after irradiation. Hence, Sertoli cells repair DNA damage, whereas other quiescent somatic testicular cells do not. This repair may be accomplished by nonhomologous end joining via XRCC1 and PARP1. In conclusion, cell kinetic and radiobiological data indicate that Sertoli cells more resemble arrested proliferating cells than the classic postmitotic and terminally differentiated somatic cells that they have always been assumed to be.