Human peripheral blood and HeLa cells were irradiated in vitro at the ultrasonic frequency of 65 kHz. The whole blood and HeLa cell suspensions were exposed to continuous and pulsed ultrasonic power levels of 0.12, 0.16, 0.72, 1.12 and 2.24 W for a period of one minute. The method of ultrasonic irradiation was carried out with the whole blood or HeLa cell suspensions coupled directly to a cylindrical transducer while heating of the cell suspensions in excess of 41 degrees c was avoided. Irradiated and unirradiated peripheral blood lymphocyte chromosome cultures were prepared and scored for selected numerical and morphological abberrations. There was no significant difference in the frequency of chromosomal aberrations between irradiated and unirradiated cells. The fraction of cells S surviving after an exposure to an ultrasound dose of D can be represented by S = exp (-D/Do). Further, Do is shown to depend on the time after exposure at which survival is assayed.