Ultrasounds were described by a few authors as possibly inducing sonodynamic reaction, with singlet oxygen production, as photodynamic therapy. The aim of this project was to evidence this effect and to try to explain its different mechanisms. A specific device was developed with a strict control of temperature to avoid hyperthermia and of acoustical intensity: the characteristics of the US beam and the reproducibility of treatment conditions were strictly evaluated. We studied the frequency of 2.21 MHz using an antiresonance frequency of a transducer. US treatment was applied continuously or in pulsed mode. Human colorectal adenocarcinoma cells (HT-29) were used to test the cytotoxicity using trypan blue exclusion test. Analyses were performed using cell suspensions. Different intensities were studied ranging from 0 to 3.7 W/cm2. Moreover, fluorescence emission spectra of hematoporphyrine derivative (HpD) were recorded before and after US treatment. Results of viability showed a higher cytotoxicity with US alone or with HpD in cell suspensions from 3.7 W/cm2 (20% survival). These results show that cavitation alone can account for the cytotoxic effects of sonotherapy. In fact, cavitation is higher with continuous than with pulsed US treatment. No significant difference was found with or without HpD. HpD fluorescence spectra did not differ before and after US treatment suggesting that no modification of HpD structure was induced by US. Fluorescence spectra showed a very slow and small decrease in fluorescence intensity with time probably caused by the low interfering light used for the experiment. In conclusion, in our experiments, ultrasounds do not seem to induce any chemical reaction with photosensitizers, conversely to what was already reported. However, other photosensitizers, molecules and different cell lines (less resistant) must be studied in order to conclude about the absence of cytotoxicity of this technique.