in individuals who were exposed at younger ages, yet the globin from 33R, who was exposed at 1 year of age, had a low content of isoleucine. Radiation-induced lesions may become fixed more frequently in infants than in adults owing to a reduced opportunity for excision-replication repair when a higher percentage of stem cells is dividing. These observations are consistent with findings that the induction of leukemias is higher in prenatal and younger persons exposed to X rays® and radiation from an atomic blast’, respectively; however, more data are needed to establish the extent to which radiation causes a higher isoleucine substitution frequency in the hemoglobin of infants than adults. The elevated isoleucine substitution frequency could arise by at least two mechanisms; one is by an increase in translational errors and another is by radiation-induced, base substitution mutations in erythropoietic stem cells. If the elevated iscleucine content in hemoglobin of exposed individuals is caused primarily by translational errors, both the a and g chains should have a higher isoleucine content than corresponding controls. On the other hand, base substitution somatic mutations should produce a higher isoleucine content in the polypeptide chain that is the product of the mutated gene because its mRNA would now code for isoleucine. These alternatives may not be readily distinguishable in cases where clones of cells with mutant o and 2 chain polypeptides appear in the same person. The isoleucine substitution frequencies in the separated a and 8g chains should equal the values obtained for the globin; this expectation was found (Table 2) but the substitution frequency was consistently higher in the g than in the o chain both in controls and