Abstract Previous work has shown that a high yield of genetic damage can be recovered from stem spermatogonia exposed to a high (900 R) X-ray dose, despite extensive cell killing, when this follows 24 h after a smaller (100 R) radiation exposure. This differs from the response of the normal stem-cell population and has been interpreted to mean that the more radio-resistant cells surviving the first exposure become sensitive both to radiation-induced killing and genetic damage after this time interval and, as a consequence, lose the heterogeneity in radio-sensitivity that typifies a normal stem-cell population. Similar results have now been obtained with doses of 600 and 800 R given in fractions of 100 + 500 R and 100 + 700 R 24 h apart. Yields of translocations among spermatocytes were higher than obtained with the single doses and responses consistent with the fractions acting additively were obtained when the fractions were given in reverse order. Further analyses of the data provided support for the concept that 24 h after a radiation exposure there is a loss of heterogeneity in radio-sensitivity in the surviving stem-cell population.