descend through the troposphere to the ground with a half time of 5 days. The besis for this estimate is unknowm. It was transmitted by personal communication of R. Dudley of the Atomic Energy Commission, Division of Biology and Medicine. There is no known mechanism for bringing very finely divided particulate matter down through the temperature inversion which marks the tropopause, so that the reservoir of strontium-90 and other bomb debris now in the stratosphere may remain there much longer than the British estimate would indicate. The third parameter cited in the SUNSHINE model concerns availability of strontium-90 in bomb debris for transfer to the bio- sphere. Most experimentalworkthusYarindicatesthatthestrontium- 90 is readily available and moves more rapidly than expected through the biospheric chain. The data does not actually permit one to state the degree of availability of all the strontium-90 deposited but only that some unknown fraction is readily available. The possibility still exists that a certain proportion of the total strontium-90 created in a detonation may be trapped in insoluble particles and thus lost insofer as plant or animal uptake is concerned. There are some indications in the work at Hanford and U.C.L.A. on soil and crops that with passage of time strontium-90 is complexed with soil and rendered less available for biospheric uptake. No quantitative statements can be made as yet regarding this facet of the problen. Thefourthandfifthparameters are essentially concerned with the Problemofconcentration and with its stable isotope. e of strontium-90 The determination of the amownt of stable strontium in the soil is being worked on but firm values are not yet available. Uniform mixing of strontium-90 with stable strontium has been shown to be an invalid assumption for at least that part of the strontium-90 which has moved through the biosphere more rapidly than expected. Whether the remaining strontium-90 will mix and be traceable quantitatively is still unknown. Thelast parameter concerns the strontium content of human eed 119