In this study, we investigated the induction of apoptosis by ultrasound in the presence of a photochemically active chlorin, mono-l-aspartyl chlorin e6 (NPe6). HL-60 cells were exposed to ultrasound for up to 3 min in the presence and absence of NPe6, and the induction of apoptosis was examined by analyzing cell morphology, DNA fragmentation, and caspase-3 activity. Cells treated with 80 microM NPe6 and ultrasound clearly showed membrane blebbing and cell shrinkage, whereas significant morphologic changes were not observed in cells exposed to either ultrasound alone, at the same intensity, or NPe6 alone. Also, DNA ladder formation and caspase-3 activation were observed in cells treated with both ultrasound and NPe6 but not in cells treated with ultrasound or NPe6 alone. In addition, NPe6 substantially enhanced nitroxide generation by ultrasound in the same acoustical arrangement. Sonodynamically-induced apoptosis, caspase-3 activation, and nitroxide generation were significantly suppressed by histidine. These results suggest that the combination of ultrasound and NPe6 sonochemically induces apoptosis as well as necrosis in HL-60 cells. They further suggest that some ultrasonically-generated active species, deactivatable by histidine, are the major mediators to induce the observed apoptosis.