forms on carboxymethylcellulose (Whatman CM23) using a nonlinear gradient of 1.7 liters of 50 mM sodium phosphate buffer at pH 5.8 and 1.0 liter of 50 mM Na,PO0,. Globin was prepared in acidified cold acetone™ and the o and g chains were separated by chromatography on Whatman cmazref 9, The protein was hydrolyzed in 6N HC] for 21 hrs at 110°C. Two percent of each hydrolysate was used to determine by amino acid analysis® the quantity of globin or separated chain in each sample. Tracer amounts of L-leucine-!*¢ and L-isoleucine-4-57H were added to the remainder of the hydrolysate; amino acids in the hydrolysate were separated on a preparative ion exchange column (1.9 X 60 cm) of 8% sulfonated styrene divinylbenzene copolymer (Beckman, Type 150A}. The radiotracers were used to locate fractions containing isoleucine and excluding leucine and also to calculate the percentage of the isoleucine eluting from the preparative column which was actually pooled for the quantitative analysis of isoleucine in the protein hydrolysate. A Beckman Model 120C amino acid analyzer was used for amino acid analyses. 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 the other amino acids in each sample. Results The substitution frequencies of isoleucine for other amino acids in the 25 samples of globin are shown in Table 1. The calculated average exposures to gamma rays and the age at exposure are indicated (Table 1, columns 2 and 3). Eight and 5 of the samples were from persons exposed to 175 and 69 R, respectively; 12 samples were from age- and sex-matched controls. Data in