CL ADS AMAR. Se AAR deltapen ke Popr/Bartirt /tlinscH/ConrRap 212 leucine. Isoleucine-"H radioactivity counts were also used to calculate the percentage of isolcucine eluting from the preparative column which was actually pooled for the quantitative analysis of isoleucine in the protein hydrolysate; i.c. the small quantity of isoleucine that did not completely resolve from !eucine could be included. in computing the quantity of isoleucine in the original sample. The pooled eluate must not contain much leucine because it would interfere with an accurate determination of the small quantity of isoleucine there. After adjusting the pH to 2.2 by adding 1 N HCI, the quantity of isolcucine in the pooled eluate was determined using a Beck- man Model 120C aminoacid analyzer [11]. The frequency at which isoleucine substitutes for other amino acids in human hemoglobin was calculated by dividing the nanomoles of isoleucine by the nanomoles of all other amino acids in the sample. About 3%of the total protein, mostly molecules of higher molecular weight, were separated by molecular seiving over Sephadex G—-200. Based on an average isolcucine content of 5% for nonhemoglobin proteins, the carbon monoxyhemoglobin fraction still contained 34% nonhemoglobin protein after molecular seiving. This was removed by repeated chromatography of chemically derivatized hemoglobin over carboxymethylcellulose. The substitution frequencies of isoleucine for other amino acids in the highly purified hemoglobin from 15 controls of ages 20-60 years are shown in table I, Data on 13 Marshallese who were exposed to fallout radiation in 1954 are presented in table I to indicate that the method does detect indivi- duals who have hemoglobin with a higher isoleucine content, presumably due to radiation-induced base substitutions in erythropoietic stem cells. One of the controls, sample 1547, showed a significantly higher value than the rest and it was not included in making the following calculations. The average isoleucine substitution frequency for the 3 laboratory personnel and the 12 Marshallese controls is 2.99 x 10-*, and the computedline of best fit has a positive slope of 0.0296 x 10-*/year. The isolcucine substitution frequency in the separated a- and @-chains of a few samples was determined to show that the values equalied those obtained for the globin, although the substitution frequency in the B-chain was highcr than in the a-chain (table I), 2? . : themsiad Ot Results