Human lymphocytes were exposed to an ultrasound pulsing regimen similar to that typically employed in clinical Doppler measurements of fetal blood flow, to determine whether such pulses were able to alter the frequency of sister chromatid exchanges (SCE). A 3.1‐MHz, 1.5‐cm‐diam transducer emitted 5‐ to 50‐cycle pulses in a beam that was directed vertically along the axis of a stationary tube containing the blood suspension. Hydrophone measurements within the tube showed that the beam intensity profile was maintained beyond the free field focal region, thereby ensuring maximum exposure of the contents of the tube. A significant increase in SCE frequency was observed in blood from two random donors exposed to “diagnostic” levels of ultrasound, where the SPTA intensity ranged from 1 to 4 W/cm2. However, a number of further studies on blood from a single (different) donor have failed to show any effect from ultrasound even after an exposure dwell time of 24 h. This extraordinary finding may have some relevance towards explaining some of the controversial reports of SCE induction in human lymphocytes. Research continues to try to identify the mechanisms responsible for this effect.