Contamination from Local Fallout earth group together, 1.2 we. The contnbution of this amount of internal contamination 1s small as compared to the 173 r external gamma that the Marshallese people received. In the first few months following exposure, Sr8* and I!8! (plus the shorterlived iodine isotopes} contributed the greatest internal radiation dose. In the acute exposure, }?** and the shorter lived 18°, 113% and 1785 contributed the highest individual tissue dose, 100-150 rep to the thyroid. Sr®° contnbuted the major portion of the beta dose to the skeleton at this early time. Estimation of the Internal Radiation Hazard The potential radiation effects produced by specie quantities of internally deposited radioisotopes can be predicted from the clinically observed effects of known amounts of internally deposited radium. These effects do not appcar. of course, until a penod of years has elapsed. and observations made at earlytimes following exposure vicld no significant data concerning the damage. Thus. as antiaipated. the people and ammai!s on Rongclap Isiand who recencd au mtermediate fallout dose exhibited only transitory changes in blood-cch levcis over the 5-vear period of observation since their exposure, but no other pathological changes that could be ascribed to radiation. In terms of chromic exposure toe internal radiation, Sr" is clears the critical element. Particular cfort was therefore made to determine its icvels im soil, Cs!87 has a gaseous precursor with a halt-life sufficiently long to avoid early condensation in the fireball. Cs?87 thus follows Srinto the stratosphere. Since the fission yields and the half-lives of the two radioelements are nearly equal, thev are present in the fallout in like quantities. While thev behave differently in the ecological cycle, based on their differing chemical properties, Cs!37 nevertheless provides a useful tracer for studying the movement of Sr® through the biosphere, since its gamma-emitting properties make it readily detectable. There is also some interest in the neutron-induced radioelement Zn®%*, even though it, too, does not appear in levels which are hazardous to human beings. The interest centers chiefly around the fact that it is definitely transmitted through marine life, and thus provides a clear example of the transmission of a radioclement through the food chain to man. Other fission products also appear in small amounts in the food chain and thus appear ultimately in man, but their Jevels are so small that little effort 319 we even though of minor sigmficance as an interna] radiation hazard. Like Sr®, ~ testing, the sustained high level of the shorter-lived Sr8® may result in a hazard of the same order of magnitude. Thus Sr8* has been a significant fission product up to the cessation of testing in 1958. Of the gamma-emitting fission products, Cs!37 is of the greatest interest, ne ny plants and man himsclf, and also the inter-relationship of these levels. In the situation where people are exposed to additional fallout from continued weapon