misrepair of sublethal radiation-induced biological damage that can result in gene mutations. Such mutations can increase an exposed population’s risk of heritable genetic effects when theaffected genes are in the reproductive cells, or of neoplastic effects in the case of somatic cell genes. The disease outcomes of these lesions are not unique to radiation. Their association with radiation has been identified only by observation of. increased rates of the end-points in experimental and epidemiologic studies of irradiated populations. In the absence ofdefinitive epidemiological data for stochastic effects in the low dose range (<200 mGy)’,it is assumed for radiation protection purposes that there is no dose below which no biological damage occurs. This type of injury may be clinically expressed at random among a population exposed to radiation, (somatic effects) or their progeny (genetic effects), with the probability of the risk of clinically detectable outcomes increasing above that of spontaneous (background) risk in the non-exposed population with increasing dose above zero. However, unlike deterministic effects, the severity of stochastic effects is independent of dose. Exposure of the pregnant woman to radiation can induce non-specific deterministic effects in the embryo or fetus that are related to dose, dose rate and the period ofgestation. However, the results of experimental and epidemiological studies show that external whole-body doses of 50 mGy orless to the embryo or fetus at any time during gestation are not associated with significantly increased risks of such effects when compared with those not so exposed. It has been suggested that the fetus may be more susceptible to radiation-induced cancerin laterlife but there is no strong epidemiologic evidence of this (26). * mGy = milligray = .001 Gy; International System (SI) unit of radiation absorbed dose. 10 mGy = 1 rad (conventional unit); 1 Gy = 100 rad (conventional unit). SI units are used throughout the text except in citing original data that were reported in conventionalunits; in these instances the data are presented in SI units with the published form of the data following in parentheses. 4