Abstract
1. Serial marrow studies were performed during the first few days in eight men accidentally exposed to a mixed neutron gamma irradiation. They showed the occurrence of a wave of cytologic abnormalities. These were identical with those seen in animal experiments 1-3 days after whole body irradiation. They were considered to be "mitotically connected" (M. C. Abn.) and included the occurrence of chromosomal bridges and chromosomal fragments in mitoses. In interphase cells, the main abnormalities were nuclear fragments ("karyomeres") in the cytoplasm of erythroblasts, myelocytic cells and lymphocytes; bi- and multinucleated cells; and giant cells. The peak of abnormalities in the erythropoietic forms was reachéd after 2 days; that in the myelopoietic cells 4 days after exposure. On the 4th day, there was a distinct dose-dependent difference in these abnormalities between the high dose group (236-365 rads) and the low dose group (22-68 rads).
2. Some cytologic abnormalities, as seen in increased regeneratory activity of the marrow, were found in marrow smears 3.5 years after the accident, although the peripheral blood counts and mitotic indices of the marrow were within normal range. Their significance is obscure.
3. A careful cytologic evaluation of serially aspirated marrow samples during the first hours and days after whole body exposure proves to be an additional important aid in the assessment of the exposed individual and may well prove to be useful in determining the degree of injury and thus the prognosis.
