radiation from radium dose in humans, per microcurie, is twice that in mice due to exhalation of less radon and that therefore the strontiun- 89 to radium ratio in humans should be 1:20. Since strontium-90 has two disintegrations in its decay chain to one for strontium-89, this ratio is set at 1:10 for the strontium-30 to radium in humans. The maximum permissible concentration of xadium on a microcurie basis is 0.1 microcurie; therefore, the maxin um permissible concentration for strontium-90 has been set at 1.0 mi.crocurie. Since strontium-89 might be expected to follow the same pathway through the biosphere as strontium-90, it is instructive to examine the possibility that the former isotope may be an important factor in the over-all strontium problem. Fall-out studies to date tend to indicate that a short-cut may exist in the soil to plant to animal cycle; further, that some if not most of the animal ingested strontium comes from the mechanical deposition of fall-out upon the exposed surfaces of the leaves of plants rather than through a complex soil to plant nutrient cycle. This, coupled with the fact that cows' mllik, the main source of radio active strontium, is consumed in a matter of several days, can appre ciably shorten the previously estimated length of time between fission product formation and human bone deposition. It is possible, allowing one week for fall-out to occur, a second week for strontium to be ingested by a dairy cow, and a third week to be ingested by man, that deposition in bone could begin to occur in detectable amounts within a period of several weeks after a bomb detonation. A consideration of these two isotopes of strontium in terms of various controlling factors may be summarized briefly as follows: 1. Strontium-89 and strontium-90 are produced in amounts of approximately 1 gram each per kiloton fission yield. 2. These radioisotopes are biologically available and can follow the pathway of calcium both in the plant through animal to man cycle and in metabolic deposition in bone. Lol