The present study was performed to elucidate the mechanism of enhanced hyperthermic cell killing by a non-thermal effect (cavitation and direct effect) of ultrasound under various gas conditions. Cavitation, as indicated by formation of DNA double-strand breaks and liberation of potassium iodide, was completely inhibited under N2O-saturated conditions, while it was promoted under O2-, Ar-, and N2-saturated conditions. Mouse L cells were treated with ultrasound (1 MHz continuous wave, spatial peak temporal average intensity; 3.7 W/cm2) and/or 44 degrees C hyperthermia in medium saturated with O2, Ar, N2 (with cavitation) or N2O (with direct effect). The synergism on cell killing by ultrasound and 44 degrees C hyperthermia was observed under N2O-saturated conditions (enhancement ratio = 1.39). On the other hand, additive enhancement was observed under O2-, Ar-, or N2-saturated conditions. In addition, when cells were treated with 44 degrees C hyperthermia before or after sonication under N2O-saturated conditions, synergistic cell killing was not observed. These results suggested that the direct effect of ultrasound alone did not influence cell killing, but enhanced the hyperthermic cell killing synergistically, when both agents simultaneously acted on the cells.