The extent to which human somatic and germinal mutation frequencies may be increased by exposure to ionizing radiation and to the variety of chemicals that characterize modern societies has been a matter of concern in recent years. Somatic mutations, either genic or chromosomal, are not transmitted to the offspring of an exposed individual. However, increases in the frequency of these mutations may contribute to an increase in the frequency of acquired disorders, for example, cancer. Increases in the frequency of germinal mutations, genic or chromosomal, are likely to contribute to inherited defects in the offspring of individuals exposed to mutagenic agents. There is, therefore, a clear need to develop and apply methods to study exposed populations at risk of increased levels of Somatic or germinal mutations.