Abstract: Objective To investig at e the effect of an echo-contrast agent on the proliferation of rat smooth muscle cells under different ultrasound conditions.Methods The vascular smooth muscl e cells of rats were cultured with echo-contrast agent in 96-well plates, follow ed by exposure to ultrasound of different conditions. Trypan blue staining was p erformed 48h later, and the proliferation of the cells observed by MTT assay.Results No cytotoxicity was found by trypan blue staining when the me-chanical i ndex of ultrasound was below 0.75.Compared with the control cells, the prolifer ation of the smooth muscle cells was decreased following the exposure as the me chanical index of ultrasound increased. The most obvious inhibition of cell pro liferation was resulted when the microbubble concentration was 20% for a 60-seco nd exposure.Conclusions The inhibi-tion of the vascular smooth muscle cell prol iferation by echo-contrast agent destruction is correlated with the mechanical in-dex of the ultrasound, concentration of the echo-contrast agent, and exposure time of ultrasound.