A fibroblast cell line (3T3) and normal rat peritoneal fluid cells were exposed in vitro to pulsed ultrasound from a diagnostic instrument (Smith-Kline "Ekoline 20"). We report here on ultrastructural changes in both cell types and on altered motility patterns in 3T3 fibroblasts. Abnormal motility was detectable 10 generations after exposure. X-irradiation and ultraviolet light elicited similar effects on cell motion. It is suggested that the cellular effects of diagnostic levels of ultrasound be further examined both in vitro and in vivo.