2.1 Introduction Wuen Tue Exposep groups werefirst seen at Kwajalein after evacuation from their native atolls, the amount of radiation they had received was not known with certainty. It was skin lesions, epilation, granulocytopenia and thrombocytopenia. The skin lesions were first observed between the 12th and 14th post-exposure days. These lesions were most prevalent taken at the sites of the fallout and from monitoring all individuals, that a significant amount extent in Group III. Details of the skin symptoms and lesions and their treatment are reported in Chapter 3. Details of hematologic studies are presented in Chapter IV. Granulocytopenia and thrombocytopenia of marked degree developed in many individuals of Groups I and II and was of sufficient severity to warrant serious consideration of prophylactic and therapeutic measures for potential sequelae of these cellular deficiencies. In view of the conflicting opinions about the value of prophylactic and therapeutic measures such as antibiotics and whole blood transfusions known, however, from instrument readings of penetrating irradiation to the entire body had been received and that extensive contamination of the skin and possible internal deposition of radioactive materials had occurred. The nature of the irradiating material and thecircumstances of exposure prevented a precise evaluation of dosage (see introduction). Even if the precise dose had been known it would not have been possible to predict the biological effects since the quantitative response of man isnot known. Accordingly, a complete medical history and physical examination was obtained on each individual and numerous follow-up eXaminations were carried out. In addition, routine sick-call was held twice daily and inspection of the skin of all individuals was made at frequent intervals. Medical care was avail- able at all times. Hospital facilities were avail- able at the Kwajalein Naval Dispensary, and support by the more extensive medical facilities of the U. S. Pacific Fleet had been promised if needed. From descriptions of the amount of fallout material and from radioactivity measurements, it was apparent that Group I (Rongelap) had received the highest doses of radiation, Group II (Ailinginae) and Group III (Americans) an intermediate amount and Group IV (Utirik) the least. From physical dosimetry it was later estimated that Group I had received approximately 175 r of gamma radiation; Group II, 69 r; Group III, 78 r; and Group IV, 14 r. The most seriousclinical and laboratory mani- festations of irradiation appeared in Group I and IT. The only abnormalities that could be attributed with certainty to irradiation were in Groups I and IT but were present to a slight in the treatment of radiation disease (1-5), it was decided that therapy would be instituted only as indicated clinically for specific condi- tions as they arose. In order to determine the effect of the internal deposition of radioactive material on the course of the externally induced radiation injury, it was necessary to determine the degree of internal radioactive contamination. Details of the measurement of internal deposition of radionuclides are considered in Chapter V. It is sufficient to state here that the contribution from the internally deposited radionuclides to the total acute dose was insignificant. 2.2 Symptoms and Signs Related to Radiation Injury Severa Symptoms Titat developed during the tirst two days could be attributed to radiation. These symptoms were associated with the skin and the gustrointestinal tract. Itching and burning of the skin occurred in 28 pereent of Group I (Rongelap), 20 percent 15