Ultrasound contrast agents comprised of microbubbles less than 5mum in diameter have been demonstrated to scatter subharmonic energy at one half the driving frequency. At ultrasound frequencies in the 20-40MHz range, the subharmonic offers the potential to differentiate the blood in the microcirculation from the surrounding tissue. It is unknown whether current contrast agents, manufactured to be resonant at clinical frequencies, are ideal for subharmonic imaging at higher frequencies. Numerical simulations of the Keller-Miksis model and experimental investigations of Definity microbubbles support the hypothesis that off-resonant bubbles, excited at their second harmonic, may be primarily responsible for the observed subharmonic energy. The optimal bubble size and transmit conditions for the generation of coherent subharmonics in vitro were also determined. Definity is a suitable ultrasound contrast agent for subharmonic imaging at 20MHz with peak-negative pressures between 380-590kPa and pulses greater than or equal to 4-cycles in duration.