The amount of natural strontium in the human body is a quantity we know rather well. It has been carefully measured and is about 0.7 gram in the average adult, with proportionately less in children. Now since we know how greatly Sr⁹⁰ has been diluted in the soil and how much natural strontium there is in our bodies, we can calculate the expected quantity of Sr⁹⁰ in our bones. Considering the many uncertainties in the calculation one should not expect too good an agreement. The remarkable fact is that the quantity of Sr⁹⁰ measured in small children does agree with the calculated amount. For adults the measured value is quite a bit less than the calculated amount because adult bones have been made for the most part before there was any Sr⁹⁰ in the environment. The fact that we can calculate how much Sr⁹⁰ is at present in the body is most important because it gives us confidence that we understand what is happening. It is especially important for us to understand what is happening so that we can predict how nuclear tests which are carried out today will affect future levels of Sr⁹⁰ in the body. From arguments such as we have given, plus a record of the Sr⁹⁰ content of bones over the last several years, it seems unlikely that the level of Sr⁹⁰ will increase by more than a factor of two or so due to tests already conducted. Actually this factor may be even smaller both because of the mixing of the strontium with the deeper layers of the soil, and because the radioactive strontium which stays in the ground for a long time tends to become chemically less soluble and mixed more thoroughly with that part of the natural strontium which is chemically unavailable. This latter process is called “chemical aging.” To follow radioactive strontium and normal strontium from the soil into the food and the bones is not an easy matter. We must worry about the question of the strontium depth in the soil and the chemical form of the strontium. The complete identity of Sr⁹⁰ and normal strontium holds only if both are near the same place and in the same chemical form. A further difficulty is that until recently little was known about the behavior of normal strontium and knowledge is accumulating slowly. Much more is known about calcium. Now calcium and strontium do not behave in an identical way, but they do behave similarly. In passing from soil to man the ratio of calcium to strontium does not remain the same but at least it changes in a more or less definite manner. Actually most work on Sr⁹⁰ uptake has been done by comparing Sr⁹⁰ with calcium. In order to use the data on calcium one has to find out how the calcium to strontium ratio is changed when the material is taken up into the human body. In the soil there is, on the average, about 1 part of strontium to 100 parts of calcium. In the human body the ratio is about 1 to 1400.