Microbubbles were isolated by size using differential centrifugation, and the effect of microbubble size on the acoustics and kinetics of contrast ultrasound imaging was measured in the mouse kidney. Microbubbles in the 1-2 μm diameter size range comprise over 90% of the microbubbles in freshly generated and commercially available lipid-coated microbubbles. These smaller microbubbles attenuated strongly without producing much backscatter, thus behaving like a negative contrast agent. The high degree of ultrasound absorption may be due to strong damping by the shell. On the other hand, microbubbles in the 4-5 and 6-8 μm diameter size ranges were highly echogenic. Also, larger microbubbles exhibited longer contrast persistence, indicating that dissolution is the main mechanism of elimination. For example, 6-8 μm diameter microbubbles circulated for greater than 15 minutes at a dose of 5x10∧8 microbubbles/mL in a 0.1-mL bolus. These results demonstrate that the magnitude and duration of contrast enhancement is strongly dependent on the microbubble size distribution.