The sonodynamically induced antitumor effect of Photofrin II (PF), was evaluated in mice bearing colon 26 carcinoma. In order to find the optimum timing for ultrasonic exposure after the administration of PF, the PF concentrations in the plasma, skin, muscle, and tumor were measured. The antitumor effect was estimated by measuring the tumor size. Since the highest concentration of PF in the tumor was observed 24 h after administration, an ultrasonic exposure timing of 24 h after the intravenous administration of PF was chosen. When used alone, ultrasound showed a slight antitumor effect, which became increasingly significant as the dose of PF was increased, while use of PF alone showed no significant effect. From these results, it is concluded that PF significantly sensitizes solid tumors to ultrasound, demonstrating a synergistic antitumor effect.