Metaphase chromosomal aberrations were produced by tritium-labeled thymidine and by tritium-labeled uridine incorporated into human peripheral leukocytes in vitro. The distribution of aberration types is different from that of aberrations induced by X rays, and one type of lesion was produced by tritium that has not been seen in material treated with X rays. No correlation between the distribution of chromosomal breaks and the distribution of label in autoradiographs was observed, although more aberrations occurred, as expected, in heavily labeled cells. The breakage frequencies observed correspond to single acute doses of up to about 100 r of hard X rays.