showing minimum count at 4th- 8th week. 5 cases revealed a count of less than 2,000/mm>, 13 less than 3,000 and 5 less than 4,000. In one case, the leukocyte level depressed to 800. A correlation was found between these minimum count and the externally irradiated gamma doses of each individual. At first lymphopenia was noticed and then neutropenia became marked. proved. Since 8th week recovery was In many cases remarkable eosinophilia was observed at that time. In some cases immature neutrophils appeared in peripheral blood slightly. b. Erythrocytes : In severe cases slight anemia was observed, accompanied by the depression of reticulocytes. were higher than 1.0. Color indices The Price-Jones curves of erythrocytes diameter were displaced to the right of normal one at first, which returned to almost normal after one year. c. Platelets : Platelets counts showed increasing depression, reaching minimum at the 4th- 7th week ( 15,000 - 100,000/mm’). Slight eoagulation disturbances observed in a few cases. d. -Bone marrow: In severe cases bone marrow was highly hypoplastic at the critical stage, which changed to hypoplastic and turned into almost normoplastic. after a year. Recovery was not complete even At the recovery stage coexistence of hypoplastic area and hyperplastic area was observed in histological sections. e. Morphological abnormalities : Several morphological abnormalities, e.g. abn¢rmal granules in lymphocytes or neutrophils, vacuoles in various leukocytes and megakaryocytes, giant nuclei and hypersegmentation of neutrophils, binuclear lymphocytes, abnormal mitosis of erythroblasts etc. were observed for about one year, especially at the critical and recovery stage. A little increase of "mitotically connected abnormalities'' was found in bone marrow smears of a few cases after 10 years. £. Recovery: The cumulative distribution curves of numbers of leukocytes, erythrocytes and platelets displaced to the left of normal - 91 ~