stitute the critical animal pathway leading to deposition of strontium- 90 in human bone. This pathway may be expected to yield hazard levels to a significant population before alternate eating habits would be hazardous in other populations. Experiments performed on cowr indicate that the relative strontium-90 to calcium ratio in feet is reduced to 0.13 of its value by the time the strontium reaches tne milk. The SUNSHINE Project data correlating the strontium-90 to celcium ratio in cows' milk with the ratio existing in their alfalfa feed indicate a reduction to approximately 0.15. This reduction is quite possibly brought about by the avidity of the bones of the cow for strontium-90. This avidity and hence the reduction of strontium-90 to calcium ratio in the milk might well disappear if the bones had been formed in an environment with the seme strontium-90 to calcium ratio as was being fed at the time the milk was formed. For long-term hazard calculations, it is felt that this apparent factor of reduction in strontium-90 to calcium ratio should be ignored. Current interpretations of the data, therefore, should raise the values found in milk relative to that found in the animal feed when making hazard estimates for high concentrations of strontium-90. For the same reason a reduction in the strontium-90 to calcium ratio in fetal bones over that existing in the diet should not be used to support the possibility of a placental barrier in the human as a protective device for the part of the bones laid down in utero. Since the calcium laid down in fetal life constitutes about 2% of the total potential skeletal calcium, this phenomenon of placental barrier protection does not appear to be important quantitatively. The possi- bility of increased sensitivity to radiation in infant life, however, may magnify this beyond the proportion indicated by comparison with the final total of calcium in the adult skeleton. The intake of strontium-90 by humans comes from four sources: air, water, plants, and animal products. The amount of strontium-90 provided by inhalation of air 106