Monkeys, rabbits, dogs, rats, mice and cats received radiation doses ranging from 25 to 1500 r usually given as total body irradiation in one exposure. The observation time extended up to the 25th day, with first counts at 24 hrs. and thereafter at 48-hr. intervals up to the 11th day. Erythrocytes exhibited no changes up to 200 r, but 300-500 r produced questionable changes up to 168 hrs. A substantial reduction of erythrocytes was found between 168 and 448 hrs. No changes in the reticulocytes were observed with doses up to 100 r; 200-500 r produced well-marked reduction, persisting after a dose 200 r up to 120 hrs. and, after a dose of 500 r, up to 280 hrs. No. changes in the platelets were observed up to 200 r. Doses between 300 and 500 r produced a definite reduction, lasting for 120 to 168 hrs. Doses of 500 r and more resulted in reduction starting at about 120 hrs. and becoming very definite by the 5th day. Neutrophiles showed no changes up to 100 r. Transient neutrophilic leucocytosis occured in rats after 550 r. After 24 hrs. there was a definite reduction in numbers of neutrophiles with continuous decline up to 72 hrs. Regeneration started at the 12th day. A dosage of 25 r produced in rats a fall in lymphocyte count within 24 hrs. and after 550 r a reduction in lymphocyte count in peripheral blood was noted 15 min. after exposure. Reduction of total lymphocyte count was still present after 25 days in all survivors receiving more than 50 r. In rats receiving 550 r (LD50) changes in the blood-forming tissues were noticeable 1 hr. after exposure. At 6 hrs. cytolysis of cells in the lymph follicles reached a maximum. At 24 hrs. repair in lymph nodes was actively in progress and it was completed at about the 20th day. The findings in lymph nodes did not show a good correlation with the peripheral blood picture; the total white count was still reduced at the 25th day. Changes in the spleen followed closely those in lymph nodes, but the process was somewhat prolonged. Complete regeneration did not take place before the 40th day. The bone-marrow showed destruction 2.5-5 hrs. after irradiation. Most of the debris was removed 30 hrs. after exposure. By the 8th day severe hyper-plasia was found. Active regeneration occurred about the 12th day. Repeated small doses of 1 r/day for 1-2 yrs. represent about the minimum critical dose to produce detectable hema-topoietic changes; 10 r/day resulted in hematopoietic changes within 1 year. All doses shortened survival time of the animals. Also an increased incidence of leukemia was noted.