Murine acute myeloid leukemia is characterized by chromosome 2 aberrations, and genesis of the marker chromosome 2 by radiation is suspected to be an initiating event of radiation leukemogenesis. A detailed analysis of the type and frequency of chromosome 2 aberrations in murine bone marrow cells at an early stage after irradiation is provided here. A total of 40 male C3H/He mice was exposed to 137Cs gamma-ray at a dose of 1, 2 or 3 Gy, and sacrificed 24 hours after irradiation. Metaphase samples prepared from bone marrow cells were Q-banded for karyotyping or painted with DNA probes specific to chromosome 2. In 5 mice analyzed by karyotyping, one mouse showed high frequency of the marker aberrations as well as other chromosome 2 aberrations. Chromosome painting analysis for the rest of the mice also detected 3 animals showing significantly high frequencies of chromosome 2 aberrations. Dose-dependence of the frequencies was observed even among those mice that tended to be sensitive. The results indicated that there was a subgroup of mice carrying hypersensitive chromosome 2. The subgroup could be leukemia-sensitive if radiation-induced chromosome aberrations are responsible for an early change in myeloid leukemogenesis.