The antitumor effects of combined use of ultrasound (US) and a photosensitizer, hematoporphyrin (Hp), were determined in mice bearing sarcoma 180. In order to find the optimum timing of the US irradiation after the administration of Hp, the Hp concentrations in the tumor and in the plasma were determined and were analyzed pharmacokinetically. Antitumor effects were evaluated by measuring the tumor size and the tumor weight. Hp alone showed no antitumor effect but US alone showed a slight antitumor effect. The combined treatment with US and Hp showed marked synergistic effects on sarcoma 180 (inhibition ratio was 74% of the control). From these results, the enhancement of antitumor effect is thought to be caused by the sensitization of tumor cells to US mediated by Hp.