36 formation. Neither growth nor decreased dietary calcfum or phosphorus had an effect on the metabolism of yttrium, cerium, and plutonium. Strontium was eliminated with a half time of 3-4 months, much more rapidly than the el imination of yttrium, certum and plutonium which had half-times of 1-3 years. Work in rabbits also indicated that uptake of injected 8%Sr and 90Sr by the skeleton was greater in young animals than in old ones, and was inversely related to calcium levels in the diet (Kidman et al., 1950). . The transfer of radiostrontium from the lung into the bloodstream was also investigated in rats (Abrans et al., 1946). Immediately after 30-min exposure to 89Sr as SrCi2, more than 50% of strontium that was deposited in the Tungs had been ranoved fran the lungs, and 25% ‘was in the skeleton. The Tung burden was reduced by half over the next 30 minutes, and after 8 hours, only 2.5% of the initial burden was in the lungs. deposited. Over 85% was skeletal ly Direct skeletal uptake of inhaled weapons fallout radiostront ium in test animals was demonstrated fn 1952 and was found to be preferential ly concentrated in growing bone (Smith et al., 1952). Although the behavior of strontium was qualitatively similar to that of calcium, it was shown in experimental animals that the ratio of strontium to calcium in tissues was different fram the ratio of the two elements in the diet. To better quantify and predict the differentiation in tissues or excreta, Comar et al. (1956) proposed the term “Stront tum-Calcfum Observed Ratio” (OR), where Sr/Ca of sample ORsample-precursor * yrytg op ceareecar