In this study, we investigated the induction of apoptosis by ultrasound in the presence of the photochemically active gallium-porphyrin complex, 7,12-bis(1-decyloxyethyl)-Ga(III)-3,8,13,17-tetramethyl-porphyrin 2,18-dipropionyl diaspartic acid (ATX-70). HL-60 cells were exposed to ultrasound for up to 3 min in the presence and absence of ATX-70, and the induction of apoptosis was examined by analyzing cell morphology, DNA fragmentation, and caspase-3 activity. Cells treated with 80 μM ATX-70 and ultrasound clearly showed membrane blebbing and cell shrinkage, whereas significant morphologic changes were not observed in cells exposed to either ultrasound or ATX-70 alone. Also, DNA ladder formation and caspase-3 activation were observed in cells treated with both ultrasound and ATX-70 but not in cells treated with ultrasound or ATX-70 alone. In addition, the combination of ATX-70 and the same acoustical arrangement of ultrasound substantially enhanced nitroxide generation by the cells. Sonodynamically induced apoptosis, caspase-3 activation, and nitroxide generation were significantly suppressed by histidine. These results indicate that the combination of ultrasound and ATX-70 induces apoptosis in HL-60 cells. The significant reduction in sonodynamically induced apoptosis, nitroxide generation, and caspase-3 activation by histidine suggests that active species such as singlet oxygen are important in the sonodynamic induction of apoptosis.