"The most massive exposure of human persons to ionizing radiations took place in August, 1945, in the Japanese cities of Hiroshima and Nagasaki. The desirability of detecting any genetic effects of such exposure was recognised by the U. S. A.-sponsored Atomic Bomb Casualty Commission (ABCC Report 1956). To this end, between February 1948 and February 1954, about 65 000 births in both cities were studied; the results were reported in great detail by NEEL, SCHULL and many others. In this publication, referred to henceforth as the ‘ABCC Report’, possible genetic effects are studied in relation to maternal age, parity, maternal state of health, parents’ socio-economic status]], as well as to exposure of the parents to the bomb. For the latter factor, the births (after eliminations of multiple births and of those from consanguineous unions) are distributed in a number of ‘exposure cells’ according to a system of grades, as follows for each parent:"