The radiosensitivity of spermatogonial stem cells to X rays was determined in the various stages of the cycle of the seminiferous epithelium of the CBA mouse. The numbers of undifferentiated spermatogonia present 10 days after graded doses of X rays (0.5-8.0 Gy) were taken as a measure of stem cell survival. Dose-response relationships were generated for each stage of the epithelial cycle by counting spermatogonial numbers and also by using the repopulation index method. Spermatogonial stem cells were found to be most sensitive to X rays during quiescence (stages IV-VII) and most resistant during active proliferation (stages IX-II). The D0 for X rays varied from 1.0 Gy for quiescent spermatogonial stem cells to 2.4 Gy for actively proliferating stem cells. In most epithelial stages the dose-response curves showed no shoulder in the low-dose region.