Mouse sarcoma 180 or rat ascites hepatoma (AH) 130 cells were exposed to ultrasound (US; 1.27, 2.21 and 3.18 W/cm2; 1.92 MHz) for up to 60 s in vitro in the presence or absence of hematoporphyrin (Hp; 10, 25 and 50 micrograms/ml). The cell-damaging effects of treatments were determined by means of the Trypan Blue dye exclusion test. Hp alone did not show any cell-damaging effect, whereas US alone damaged 30 and 50% of sarcoma and AH 130 cells, respectively, at the maximum intensity for 60 s. In the presence of 50 micrograms/ml Hp, US damaged 99 and 95% of the above tumor cells, respectively. These results show that Hp increased the sensitivity of tumor cells to US.