undoubtedly is a minute contribution to the total body burden in a highly contaminated environment where the other modes of transport are permitted to operate over a period of years. The possibility that in- halation during the actual fall-out time may provide particles that will lodge in the lung is not ruled out as a source of critical damage, but as yet there are no indications suggesting that it requires more than passing notice and an awareness that it should be watched for. The water analyses of the SUNSHINE Project and other data indicate that water supplies such as reservoirs will not receive more than a normal share of fall-out based on surface area. The water-shed run-off water does not concentrate strontium-90 in the reservoir. It has also been shown that strontium-90 does not reach well water by leaching through the ground. It appears, therefore, that water will be a minor contributor to the total body burden in a highly contaminated environment. There are no direct experimental data on humans with regard to metabolism and excretion on a quantitative basis. Inferences concern- ing amounts excreted and amounts deposited in bone are derived from animal data. In an attempt to attach significance to the amount of strontium-90 found in bone, reference is made to the total amount of fission products released, strontium-90 soil deposition and various levels in the soil-plant-animal-milk chain. thus far are well below any hazard level. The actual values obtained Their projection to hazard levels and their quantitative significance will become apparent when hazerd calculations are discussed below. It is important to remember that at the end of a year probably 5% pr less of ingested strontium-90 is retained in bone. In terms of an equilibrium environment, this principle ceases to be important. Under these circumstances the ratio of strontium-90/calciumas it is taken in, corrected for any possible body discrimination between the two elements, becomes the important factor. The real situation may be a@ combination of a high initial intake followed by a continued lower 107