’. was changed slig. ly; a gas filled (Xe) prc ortional counter operating in anticoincidence with an umbrella of nine GeigerMuller tubes was used to detect the 5 .9-kev x~ray emitted in the electron capture decay of Pre, Pulses from the proportional counter were recorded in a 512-channel multichannel analyzer. The detection system was surrounded by four inches of lead shielding and the resultant background counting rate under the She photopeak was 1.7 counts min7!, Disintegration rates of Ke in the samples were determined by comparison with >°Fe electrodeposited from a standard solution obtained from the National Bureau of Standards. Correction was made for the self- absorption of the X-rays in the electrodeposited iron. Stable iron was determined colorimetrically, using o-phenanthralein as the color-forming agent (10). Body burdens were estimated by first measuring the 5S he in a known volume of blood (4-26 ml). Total blood volume was estimated using body weight and average blood volumes of 82 ml blood per kg body weight for males and 74 ml blood per kg body weight for females. The iron content of blood was assumed to be 65 per cent of the total body iron. This method of calculating total blood volume is that used by Persson (11) in his estimate of ee body burdens in Lapps of Northern Sweden. Previous estimates of body burdens Eom composite blood specimens (1,5) were made assuming that the average total blood volumes of 5 liters and that 60 per cent of the total iron is in the blood.