Although various in vitro studies have shown that low-intensity pulsed ultrasound influences cytoskeletal components and biochemical pathways, the exact biologic mechanisms are still not fully understood. In this study, we analysed the effect of therapeutic ultrasound on the endocytotic activity of human foreskin fibroblasts. Fibroblasts were incubated with two different endocytotic markers (transferrin Alexa 488 and Lucifer yellow; Sigma Bioprobes, Eugene, OR, USA). To evaluate the amount of internalized markers in sonicated and nonsonicated control cells, confocal microscopy and plate reader experiments were performed. Additionally, the structural integrity of the cell membrane was monitored by electron-microscopy. After ultrasound treatment a clear increase (1.6-fold/Lucifer yellow and 1.4-fold/transferrin Alexa 488) of fluorescent marker uptake was detected. Confocal microscopy and plate reader experiments revealed that whole populations of sonicated fibroblasts showed a significant higher fluorescence compared with cells not sonicated (p<0.05; t-test for unpaired samples). The electron microscopic analysis of the cells showed no signs of structural membrane damage or a loosening of the membrane integrity. However, an exceedingly high amount of endocytotic vesicles and clathrin coated pits were observed in the sonicated group.