2, After a single doue of radiation to the body as ac whole there= fore, the changes in the circulsting bloo. at any one time after xposure, depend to a large extent upon the length of life or rete or turnover of circulating Cel). 3« 'i, cen Look on the cir= £ species cach with its culeting celis as r@pulctiongs compe .2 3 eto is redueed to zero or own averege 1ife span. if the birth r ely wiaffected, Lab near aero by rediztion oni the adults reriin relativ au ting blood cells will bs proportional depopulation of the circula . a» tort’ iste span of each cpccies of celh. Thus, marked reduction in the popul:tion of short-lived celis will, become apvarent before marked reduction of the long-lived colic. There is tason to believe that the averse lite span of the white blood cells is a mutter of a few hours, that of the blood platelets is a matter of a few days, anu that of the red biood cell cs matter of more than 1CO days. «fter « single dose of radiation, the theoretic-lly anticipatec order of disappearance of circulnting celle would be first the white cells, second the platelets, end third the red blood cells. Indiaed, the red blood cell population might be expected to become/readuced so slowly certain desares of radiation that regeneration {active after nm shen “Cf celis):mie© occur before the reduction in numbers had attained a magnitude reacily detectesle by currgnt leboratory mothoces The fundamental demaze to perent red cell types in c 36 the bleod pis the redleser cent profoundly eq because of the longetivity "4 such 2 case could be asid te be mchcd ef the circulating red blood geld. Ife d G foreCiabion, ts affacted by ioue perent celis. A given dese ane Gil¥ slightly damage another. The abjlity of tissuss to rogererabe is ens ef grsat inpertance, of demage will be mere readily effocted ty c.pacitics for regeneration. host worthrhile obcervations on effect cf in smfimels. Detai f- 4. wo Repeir single dcseshf radiation heve ed data on the effects of ve:ying desage of wadiaticn on asét3 will be presented inthe body cf the lecture. required to preduce tigeucs vith marked The aiact amount of rediation + a civen effect on a given cell varies with different species, but the following cbservations seem reasonably wll establishode 2, The lymphocyte-(a type of white blood celi.) producing tissves are the most sunsitive to radiadfon danage, The tissre pro- ducing platelsts, red blood cells, and gronulecytzs ( the latter is unother typo of -hite blood cell) sre less sensitive and have a sensitivity boughly comparable one toths octner. Here is some evidence the rea blood cell-forming tissues may be imdermediate in sensitivity between the lymphocyte anc other bicod cells-emors sensitive than the latter, Zo less sc than the former. As was expected on the basis of l:novwledge of the life span of -