4.0 Introduction Fo..owi1ne tHe Detonation of a nuclear device at the Pacific Proving Ground in the Spring of 1954, 28 Americans and 239 Marshallese were exposed to fallout radiations. Sixty-four of the Marshallese on Rongelap atoll (Group I) received an estimated 175 r. of gammaradiation as measured in air; 18 Marshallese on Ailinginae atoll (Group II) received 69 r.: 28 Americans on Rongerik atoll (Group III) received 78 r.: and 157 Marshallese on Utirik atoll (Group IV) received 14 r. Detailed history of the event, as well as clinical and internal contamina- tion findings are reported respectively in Chapters I, II and V. This chapter presents the hematological findings in the exposed individuals during the first 11 weeks, at 6 months, and at 12 months after exposure. Since it is generally agreed that the degree of change in the formed elements of the blood is the most useful clinical index of the severity of radiation damage, peripheral blood changes were relied upon as a major aid in evaluating the degree of radiation injury in each exposed individual. In addition, changes in the mean blood counts of the exposed groups were followed closely to aid in evaluating the changing status and probable prognosis of the exposed groups. Therefore, emphasis was placed on standardized systematic serial determinations in order that individual and group trends could be evaluated adequately. Since it was necessary to observe the large number of exposed individuals at frequent intervals, the number of different procedures that could be done was necessarily limited. Determinations employed were chosen on the basis of knownclinical value, and ease and rapidity with which they could be done reliably under field laboratory conditions. -Accordingly coagulation and biochemical studies were omitted. An extensive literature exists on the hematologic effects of radiation. These data, and the difficulties attendant on comparing them with the present results are discussed later in this report. 4.1 Methods Hematoroctcan Examinations INCLUDED total leukocyte, neutrophile, lymphocyte and platelet counts, and hematocrit determinations. Whenever possible, an entire exposure group was studied in a single day with 2 days occasionally required to complete the larger groups. Capillary blood, usually obtained from the finger and rarely from the heel or ear was used. Two pipettes were filled for both the leukocyte and platelet counts. From each pipette a single hemocytometer chamber was filled. All pipettes were rotated for 10 minutes, and the cells were allowed to settle for 10 minutes in the hemocytometer chamber before counting. A 3 percent acetic acid diluting fluid was used for total lenkocyte counts. The blood was diluted with 1 percent ammonium oxalate for platelet counts and counted in flat bottom hemocytometers using a dark phase contrast microscope (1). Two blood smears were made using a beveled end glass slide for spreading. One blood smear was fixed in methyl alcohol. The other was stained by Wright's method, from which a 100 cell differential count was made. Hematocrits were performed using heparinized capillary tubes. One end of the capillary tube was heat sealed and the tube was centrifuged in a capillary centrifuge at 12,500 rpm for 5 minutes. Every effort was made to maintain uniform procedures in every phase of the laboratory work. The namberof personnel changes for a given procedure was held to a minimum; personnel drawing blood from a single puncture 45