C3H/10T1/2 mouse embryo fibroblast cells grown as 1.2 cm diameter monolayers were assayed for cytotoxicity and neoplastic transformation following 30 min insonations with three types of pulsed ultrasound (US). Two of the US beams, with a repetition frequency of 1 kHz, center frequencies of 2.5 and 3.5 MHz and time-averaged intensities (ITA) of 31 and 6.7 mW/cm2, respectively, simulated biomedical pulse-echo imaging. The third US beam, with a repetition frequency of 5 kHz, center frequency of 2.5 MHz and ITA of 885 mW/cm2, simulated pulsed biomedical Doppler insonation. All three US beams exhibited negative pressure amplitudes in the range of 1 MPa over the cellular preparation. In order to provide uniform insonation for all exposed cells, all beams were unfocused; pressure amplitude variations did not exceed +/- 15% within a beam diameter of at least 1.5 cm, within which test samples of attached cells were confined. Ultrasound was delivered alone or in combination with x-rays (2 Gy, 240 kVp at 3.5 Gy/min given within 5 min before US) as a costressor. Under all treatment conditions, there was no significant affect of US on clonogenic survival or neoplastic transformation (cumulative probabilities ranging from 0.13 to 0.92).