In studying the feasibility of developing tissue-targeted contrast media that can be administered as liquids and vaporized by an external stimulus such as ultrasound, we have investigated the properties of emulsions of nanometer-size particles containing perfluoropentane (n-PFP) and 2H,3H-perfluo-ropentane (H2-PFP). We found that the ultrasound intensity required to induce echographically significant vaporization can be controlled by changing the ratio of H2-PFP to PFP and that the intensity threshold increases as this ratio increases. We further determined if our nano-particles could change their phase in colon 26 mouse tumor tissues in vivo. Ultrasound exposure under the same conditions as the above in vitro experiments was found to induce a phase shift in the particles in tumor tissues. It was also found that focused ultrasound at an acoustic intensity of 20 W/cm2 contiguously exposed after the phase shift is established in the tumor tissues induce therapeutic effects at the focus. Our results indicated the potential usefulness of the nano-particle system as a versatile agent for ultrasonic diagnosis and therapy.