a Table 1 are presented graphically in Figure 1 to show the relationship between the age at exposure and the isoleucine substitution frequency. For some of the samples, sufficient globin remained to separate the & and & chains and to determine the isoleucine substitution frequency in the separated chains (Table 2). A tracing of the chain separation of sample 24R is shown in Figure 2. The small peaks just ahead of the major 8 and a chain peaks are commonly observed (see Discussion) and for a few samples these small peaks were analyzed separately (Table 2). The average isoleucine substitution frequency as a function of radiation exposure is plotted in Figure 3. Discussion A slight, but invignificant, increase in the isoleucine substitution frequency was found is contrals between ages 20 and 51; the regression line has a slope of 0.9234 ¥ 107°. More data on older and younger persons should be collected to determine whether the isoleucine substitution frequency increases linearly with age; it has been suggested that the error rate in protein synthesis shouid increase exponentially as a function of age’. In this regard, the high value obtained for sample 1547 is interesting because she is a 60-year-old woman showing signs of senility. Except in sample 1547, the higher isoleucine substitution frequencies were in samples from exposed individuals; however, some exposed persons had values in the control range (Table 1). T tests show that the 175 R and 69 R groups are not significantly different, p>. 25; at the 95% confidence limits the former is significantly different from controls, p<.03, but the latter is not, p>.08. Figure 1 shows weve CI as co Cry that the higher isoleucine substitution frequencies were observed: more often