The nonlinearity parameter B/As of two contrast agents (Albunex and Levovist) in a saline solution (Isoton II) were measured. The results show that B/As of both contrast agents were increasing functions of bubble number densities. In general, they are much higher than that of most soft tissues. For Albunex (concentration = 1 microL Albunex: 1 mL Isoton) a B/A of 2000; and for Levovist (concentration = 6.0 mg Levovist powder: 1 mL Isoton), a B/A of 600 was obtained.