The radioprotective effects of misoprostol, a synthetic stable analogue of prostaglandin E1, on spermatogonial stem cells of C3H/HeH×101/F1 hybrid mice (3H1) were analysed by establishing dose–response relationships for stem cell killing by X-rays in mice that were pretreated with misoprostol. Spermatogonial stem cell killing was studied through determination of the percentage of tubular cross-sections showing repopulation at 10 days after irradiation. In control mice, the D0 values ranged between 1·7 and 3·6 Gy, dependent on the stage of the cycle of the seminiferous epithelium the cells were in. As found previously, proliferating spermatogonial stem cells were much more radioresistant than quiescent stem cells. In the misoprostol-pretreated animals the spermatogonial stem cells were more radioresistant, the D0 values ranging from 3·6 to 5·0 Gy. Both proliferating and quiescent spermatogonial stem cells were protected by misoprostol. As the dose–response curves in control and misoprostol-pretreated mice showed about the same extrapolation number to the y-axis it was concluded that the misoprostol pretreatment did not alter the kinetics of the repopulation process.