THE IMMEDIATE effects of radiation injury upon the hemopoietic system are well known, but few such observations have been made on humans surviving a single massive exposure. Snell, Neel, and Ishibashi1 initiated such a controlled study in 1947 on a group of subjects exposed to radiation by the atomic bombing of Hiroshima on Aug. 6, 1945. The present report is a hematologic survey of similar nature conducted from March, 1948, to February, 1949, on 824 irradiated subjects, residents of Hiroshima, and 1,145 control subjects, residents of Kure. The studies reported on 304 of the irradiated subjects represent a second examination, as the initial examination was included in the report in 1947.1 It represents a study of the hematologic effects of radiation 33 to 44 months after the explosion of the atomic bomb. It is felt that no useful purpose will be served by a review of the literature