3 uals were evacuated to the nearby naval base at Kwajalein. By this time, initial symptoms had completely subsided. An emergency medical team, composed largely of naval personnel from the Naval Medical Research Institute and the U. S. Naval Radiological Defense Laboratory, was organized and sent to the area, arriving on the ninth day after exposure. Complete initial histories and physical examinations and frequent follow-up examinations and treatment for medical conditions were carried out on all personnel. In addition, hematological studies to assay the degree of radiation damage and urinary excretion studies for radioactive materials were performed. It was apparent from initial blood studies that significant radiation had been received, and by 12 to 14 days further evidence of radiation injury was apparent in the form of skin lesions and epilation. With the exception of the development of skin lesions and epilation, physical examinations at no time revealed findings in any group that could be attributed with certainty to radiation. CLINICAL OBSERVATIONS, THERAPY, AND HEMATOLOGICAL FINDINGS Since the degree of depression of peripheral blood | elements is believed to be the best index of severity of radiation injury, systematic serial determinations were carried out. These determinations consisted of total leukocyte, neutrophil, lymphocyte, and platelet counts and hematocrit determinations. Control groups, as comparable as possible to the exposure groups in respect to age, race, sex, and background, were selected. Findings are expressed in terms of percentage of control values. Significant age and sex difference in blood cell counts were noted in the control groups, and the data are presented in accordance with the differences noted. The absolute neutrophil count (fig. 1) of both younger and older age groupsfell to a value of approximately 70 to 80% ofthat of the controls during the second week, followed by a period of fluctuation until the fifth week. At this time the beginning of a second depression was noted for both age groups, and a low value of approxi-