controls, although two hemoglobin samples, 6A and 8A, from the group of 9 indi- viduals exposed to 69 R had high contents of isoleucine. Table 1. Frequency of substitution of isoleucine for other amino acids in human hemoglobin from Marshallese. Exposure (R)- No. of samples 0 controls 25 69 175 in utero 69 in utero 9 3 1 175 O offspring Substitution frequency + SEM (x 10°) 3.09 + 0.33 39 4.77 + 0.67 5 5.24 + 0.85 p value* . 4.45 + 1.16 3.26 + 0.64 6.84 0.05>p>0.025 NS NS NS NS *Probability of significant difference from controls. Blood samples were obtained from 9 exposed persons in successive years to determine whether fluctuations in isoleucine content occurred. The data are shown in Table 2. Fluctuations noted for samples 3R, 24R, 35R, and 71R could be due to variant clones of erythrocytes arising from individual stem cells, which appear and disappear from the peripheral circulation. Such fluctuations have not been observed in specimens from normal Laboratory personnel. Table 2. Frequency of substitution of isoleucine in samples analyzed in consecutive years. Substitution frequency Exposure (R) 175 175 175 175 175 69 69 69 69 Sample number 3R 1974 19.79 24R 33R 35R 71R 6A 41A 454A 13.45 4.74 5.19 8.29 6.98 4.04 454 3.65 - 34 - (x 105) 1975 1976 1.76 6.69 . 7.00 2.16 2.17 1.45 6.29 2.56 2.17 3.23 2.85 3.10