Results of cytogenetic studies of cultured peripheral blood leukocytes and direct bone marrow preparations from six patients, 7 years after accidental total-body exposure to mixed fast neutrons and gamma rays, indicated that: (1) the modal chromosome number was 46; (2) 79 (15.4%) of the 512 peripheral leukocyte and 25 (11%) of the 227 direct bone marrow metaphases were abnormal cells; (3) 46 (9%) metaphases of the peripheral blood and 7 (3.1%) in the bone marrow had chromosomal abnormalities suggesting an X1 cell; (4) a smaller G chromosome, morphologically similar to the Philadelphia chromosome (${\rm Ph}^{1}$), was found in 13 (2.5%) and 12 (5.3%) of the peripheral blood and bone marrow metaphases, respectively; (5) dicentrics and ring chromosomes were found in the peripheral blood, but not in the direct bone marrow. These findings suggest that radiation causes chromosomaln aberrations i the cultured peripheral blood leukocytes as well as in the bone marrow cells, presumably the myeloid cells. No definite conclusion could be reached about the significance of the smaller G chromosome. An alternate explanation to the "long-lived cell" theory proposed by others to account for the X1 cells would be that a substance is produced or activated by total-body irradiation and remains capable of affecting the chromosomes for extensive lengths of time.

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