Experiments were aimed to establish whether therapeutic doses of ultrasound are liable to cause demonstrable chromosomal damage in marrow cells of the mouse. On evaluation of the chromosome aberrations no significant increase was found after treatment with three different doses of ultrasound. In contrast, our "positive control group" irradiated with 50 R of X rays showed clearly demonstrable chromosomal damage. In another group of animals treated with ultrasound plus X rays, the ultrasonic pretreatment did not increase the damaging effect of X rays significantly.