C3H/He mice develop acute myeloid leukemia (AML) after whole-body irradiation, and typical chromosome 2 deletions are found in the leukemic cells. Since the cells bearing AML-type deletions expand in the bone marrow around a year after irradiation, the delayed chromosomal aberrations could be responsible for their formation. To test this hypothesis, we have cytogenetically examined primitive hematopoietic cells of irradiated animals. Ten-week male C3H/He mice were exposed to 3 Gy x-rays and sacrificed after 1, 90 and 300 days after irradiation. Bone marrow cells collected from each animal were divided into two batches. One batch was cultured in methylcellulose medium, and metaphase spreads were prepared from each growing colony. The other batch was sorted to obtain Lin⁺ and Lin^-Sca1⁺ cells, which were analyzed with FISH for the AML-type deletions. Non-clonal aberrations in colonies in methylcellulose medium were mostly chromatid breaks, and there was no case of expansion to constitute a subpopulation in the colony. The cells from non-exposed mice also carried a significant number of non-clonal aberrations, which implies the assay itself has a perturbing effect. FISH analysis of the sorted cells detected AML-type deletions in Lin^-Sca1⁺ subpopulation at one day after irradiation, and the frequency of such cells did not decline after 90 days. While the experiments are still going on, there has been no supportive evidence for involvement of delayed chromosomal aberrations in radiation-induced AML in mice.