TII. A. HEMATOLOGIC OBSERVATIONS* Background Since alterations in leukocyte and platelet counts are among the most sensitive and reliable responses to radiation injury, careful hematologic monitoring of exposed populations was instituted immediately following the accident and has continued subsequently at frequent intervals. Ail persons were not always available for study during each field trip. Accordingly, surveys were often made on a semiannual or quarterly basis by the medical team or the resident physician at Kwajalein with the objective of performing hematologic evaluations of most exposed individuals at least annually. This allowed values for each person (as well as mean values for each exposure group) to be followed serially in comparison with values for unexposed populations and with each other over the course of time. The prompt and significant depressions in leukocyte and platelet counts observed in the combined Rongelap and Ailingnae groups immediately after the accident, and the subsequent return to stable plateaus closely approximating control mean values, are shown in Figure 1. The severity of initial depres- sions in cell counts varied among the three exposed groups, correlating reasonably well with that expected from the best estimates of their degrees of radiation exposure. In the Rongelap group, these effects were moderately severe, and it was thought that an additional increment of 50 to 100 rem probably would have resulted in the sequelae of pancytopenia (infection and hemorrhage) with the statistical possibility of death (2). The early fluctuations in neutrophil and total leukocyte counts (Figure la) apparently followed a burst of leukocytosis during the first few days and might have reflected individual responses to other injuries, such as beta burns, or to transient respiratory infections that occurred during the first month. The response of peripheral lymphocytes (Figure 1b) to the radiation injury was rapid and marked, whereas the platelets fell gradually to reach a nadir in about one month (Figure lc). Recovery required approximately one year for neutrophils and at least two years for lymphocytes and platelets. It is notable that the mean peripheral blood counts recorded in Figure 1, especially over the past 6 to 8 years, have shown less annual fluc- tuations for the combined Rongelap-Ailingnae groups than for the comparison populations. This may reflect the more variable composition of the latter occasioned by gradual attrition of the age- and sex-matched controls available for followup studies and their replacement with individuals from other population groups, primarily unexposed Rongelap residents. B. Methods Hematologic measurements for the first few years following the accident were performed on capillary blood from the finger or occasionally from the Mm hoa! C 7 C4 cry *This section was written by Drs. D.E. Paglia (UCLA) and E.P. Cronkite (BNL), with the technical assistance of Mr. W. Scott (BNL). -16-