human bone will be in equilibrium with regard to strontium-90/calcium ratios peculiar to their particular position in the chain. The continued availability of strontium-90 which has been created but has not yetfallen out, looms large as one of ouwr most important present uncertainties in the world-wide deposition problem. Since new weapons tests continue to make available new strontium-90, the question of the rate of uptake of strontium-90 into the biosphere, year after year, of that deposited at any one time is difficult to resolve. If the strontium-90 deposited is actively transported to bone only for a relatively short period of time, say one or two years, then the amount of strontium-90 which it is estimated may be deposited on ' soil without constituting a hazard can be raised considerably and, in fact, the strontium-89 would be an important contributor of radiation dose to bone on a gram-for-gram basis of produced material. The very rapid uptake of strontium-90 by cattle due to direct ingestion of fall-out material is not significant with regard to the ultimate hazard calculation in terms of large scale fall-out over a limited period of time. The cattle bones wmder these circumstances would be taking up strontium-90 rapidly for a limited time only. mechanism would probably spare the milk to some extent. This After the first cycle, the regular biospheric chain should take over. The prob- lem of continued uptake is thus more important than the short peak of uptake immediately following fall-out. This discussion has omitted serious consideration of the problem of stratospheric storage and a continued slow fall-out over the years. If this mechanism is significant quantitatively, then a larger fraction of strontium-90 produced must be considered available for rapid uptake, than is available in the early fall-out period, even though this availability in time may be many years post-detonation. It must be remembered that the current 1.0 Sunshine Unit now being found in the younger age group in this country should not neces- sarily rise with continued growth. Since this is a strontium-90/calcium 115