To determine the effects of echo-contrast agents (ECAs) on ultrasound (US)-induced apoptosis and cell lysis, human myelomonocytic lymphoma U937 cells in suspension were exposed to 1 MHz continuous waves US for 1 min at an intensity of 0.5, 1.0, 2.0 or 4.0 W/cm(2) with or without non-shell type ECA, Levovist (2 mg/ml), and shell type, Optison (1 microl/ml) or YM454 (1 microl/ml). Levovist minimally enhanced the US-induced apoptosis at 1.0 W/cm(2) while Optison and YM454 did at 2.0 and 4.0 W/cm(2), as detected by flow cytometry. Cell lysis was also augmented when Levovist was combined with US at 2.0 W/cm(2), and when Optison was combined with US at 2.0 and 4.0 W/cm(2). YM454 showed the highest rate of enhanced cell lysis at 1.0, 2.0 and 4.0 W/cm(2). Therefore, this study shows that Optison and YM454 are effective in augmenting the US-induced cell killing, but not Levovist. Another result indicates that cavitation plays a role in the augmented effects and that inertial cavitation appears necessary for Optison and YM454 to effect their actions. In addition, results show that the rate of apoptosis is lower in the presence of ECAs with higher free radical scavenging activity, suggesting a possible role for free radicals in apoptosis. These findings suggest that some ECAs have potential to be adjuncts in cases wherein augmented US-induced cell killing is needed, such as in cancer therapy with US.