Although heritable translocations are an important endpoint for the assessment of genetic risk from radiation, there has been a serious information gap with regard to their induction in spermatogonial stem cells, the most important cell stage in males for risk considerations. This led to uncertainty in estimating the magnitude of risk per unit exposure. Further, the relationship between the frequency of reciprocal exchanges scored by cytological analysis of the exposed male's meiocytes and the frequency of those transmitted to first-generation offspring needed to be re-examined. In order to fill in these gaps, two radiation studies, i.e., dose response and dose fractionation, were conducted on spermatogonial stem cells in which heritable and cytologically detected translocations were scored. The present data are by far the most extensive, to date, for heritable translocation induction in spermatogonial stem cells. The linearity of the rising portion of the dose-effect curve and the additivity of effects observed in the fractionation study allow a direct estimation of the number of transmissible translocations expected per unit exposure. Thus, the expected increase in heritable translocations per rad of acute X-rays is 3.89 X 10(-5) per gamete. The data also show a lack of consistency between cytologically and genetically scored translocations.