In an effort to search for biological significance of chromosome aberration observed in bone marrow cells and peripheral lymphocytes, the presence of transforming genes in the DNA of bone marrow cells was examined in four healthy A-bomb survivors (Group I), three with preleukemia (Group II), and nine with leukemia (Group III). In Group I exposed at 300 - 500 m from the hypocenter, estimated radiation doses ranged from 565 to 667 cGy; and randomly abnormal karyotypes ranged from 30.7 % to 48.3 %. In Group II exposed at 800 m, in which estimated radiation doses were 300 - 600 cGy, one survivor had a complicated karyotype abnormality; and in the two others, abnormal clones were partly observed. Group III, which was exposed at 800 - 2,000 m and had estimated doses of 20 - 200 cGy, consisted of acute lymphoid leukemia (one), acute myeloid leukemia (five), and chronic myeloid leukemia (three). The patient with acute lymphoid leukemia had a complicated karyotype abnormality. N-ras genes were observed not only in seven acute or chronic leukemic patients but also in three healthy survivors. This may have important implications for the mechanism of leukemic transformation. (Namekawa, K.).