Sor a) An initial rise in total white count (reflected in the neulropbhile count) may ceca. Phere after the magnitude of depression of the total white and neutrophile counts, and witht Hanits their duration are a function of radiation dose: A secondary or abortive rine in the total white count (reflected in the neutrophile or lymphocyte count) may occur, followed by a second de crease, There is little species difference in the rate of depression of the total white or neutro phile count at comparable doses; however, the mate of recovery and time for complete recovery is quite different in various species. Small animals (mouse, rat. hamster) show rehitively com plete recovery to control levels, even at doses in the lethal range, by the end of the fifth week or earlier. Data on dogsare inadequate to indicate when recovery is complete; however, return to control levels at high dose leveis had not occurred by the fifth week. Swine require 9 to 15 or more weeks for complete recovery. b}) The response of lymphocytes is essentially identical in all animal species. Depresston can be detected within a few hours, and recovery from the minimumvalues (achieved in 36 to 48 hours) requires longer than does neutrophile recovery. Lymphocytes fall to very low levels al doses well below the lethal range, and increasing dose results in no or minimal further decrease in count. Lymphocyte depression appears to have no causal relationship with acute radiation deaths, . ¢) Platelet counts have been studied most extensively in dogs."© As with neutrophiles, the rapidity and mayznitude of depression is a fuynetion of dose below the lethal range. Maximum depression oceurs by the Oth or 10th day wfth doses in the high lethal range, by the 10th to 15th day at sublethal levels. Recovery begins during the third week, but is not complete by the 30th day when most studies have been ler minatég. Insufficient data are availgble to indicate the lime required [or complete recovery. Considerable evidence including studie@ in the mouse using splenic homogenates, induced bacterial infections and spontaneodg infections have indicated that critica] neutrophile levels exist below which survival is correlated. with the absolute neutrophile count following whole body irradiation. From data on degs, it appears that survival is likely unless neutrophile counts remain below 1200 cells for a periodof time. Platelet data on dogs indicate fhat agimals with external purpura have platelet counts of 50,000 or below. a= __ -* Insufficient data on large animals are fet as yet available to quantify the extent of maxi--_ mum depression of either the neutrophile or platelet counts as a function of dose in the sublethal range. The response of the platelet count in the present study was much less subject tofluctuation than were the neutrophile or lymphocyte counts. For the preceding reasons, systematic investigation of the piatelet and leukocyte counts in large animals as a function of dose in the subjethal range are indicated. It is not possible to say at present whether severity of exposure, or of radiation damage correjates better with absolute levels @f peripheral blood count, or with degree of change from corel ov pre-exposure levels. Sonure evidence on this point can be gained by comparing the depree ol depression of the neutrophle and platelet counts in the Ailinginae and Rongerik troupes, both of whieh had essentiallythe saine calculated exposure but for which control hema lolopical vakues were considerably different (the lymphocyte count ts not suitable for comparison since deprce of depression was essentially (he same in these groups and the higher-dose Rongelap group). At the time of peak depression for each element, both the neutrophile and Platelet counts were essentially identical in terms of absolute ci , but considerably differ- ent in terms of the respective control values. Thus some evidence is afforded that absolute counts, rather than counts relative to control values, may be the more reliable index of exposure in this dose range. * 4.13.6 Approximation of Minima! Lethal Dose for Man Some indication of severity of exposure can be gleaned from a comparison of minimum in- dividual counts in Japanese groups exposed at Hiroshima and Nagasaki in which fatalities occurred. In general, a significant number of deaths was encountered only in individuals whose neutrophile count fell below 1000. In the Rongelap group 42 or approximately 50 per cent had * bagan . * Caeten * . 86