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Measurements of the retention of 47Ca and of 203Pb were made following their administration by intravenous injection. Translocation to bone was measured by y counting the feet of subjects. Uptake by bone of 203Pb was comparatively slow and extrapolation to the whole skeleton indicated that 20% of the dose had been taken up within 20 days. By time, a similar fraction of the dose had been excreted in urine. These results are consistent with Kehoe's (Ke61) long-term balance studies on 2 human subjects given stable Pb orally. Uptake by bone of 47Ca was about 1.5–2 times the amount excreted in urine. Both the uptake by bone, and its excretion in urine, were more rapid than that of 203Pb due to the greater attachment of the latter to red blood cells. However, the plasma clearance rate for Pb, like that of Sr, was greater than that of Ca.