To determine the effect of 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) on ultrasound (US)-induced cell killing, human monocytic leukemia cells (U937) were incubated at various temperatures (25.0, 37.0 and 40.0 degrees C) for 1 min in air-saturated phosphate-buffered solution (PBS) containing 50 mM AAPH before exposure to nonthermal 1 MHz US for 1 min at an intensity of 2.0 W/cm(2). Cell viability was determined by means of the Trypan blue dye exclusion test immediately after sonication. Apoptosis was measured after 6-h incubation post-sonication by flow cytometry. Free radicals generated by AAPH, a temperature-dependent free radical generator, or US or both were also investigated using electron paramagnetic resonance (EPR) spin trapping. The results showed that US-induced cell lysis and apoptosis were enhanced in the presence of AAPH regardless of the temperature at the time of sonication. At 40.0 degrees C, US alone induced increased cell killing, while AAPH alone is capable of inducing significant but minimal apoptosis at this temperature. Although free radicals were increased in the combined treatment, this increase did not correlate well with cell killing. The mechanism of enhancement points to the increased uptake of the agent during sonication rather than potentiation by AAPH. These findings suggest the clinical potential of temperature-dependent free radical generators in cancer therapy with therapeutic US.