The performance, in particular, the stability of ultrasound (US) contrast agents has yet to be assessed. An in vitro system has been set up to investigate the properties of ultrasonic contrast agents under different suspension conditions. This is designed to contribute to the optimal use of agents in clinical practice. In this study, the contrast agents were introduced into solutions of different oxygen concentration levels, as might be encountered in blood, and their relative performance was assessed in terms of decay in the solution environment, The partial pressures of oxygen in those solutions ranged between 1.5 and 26 kPa, Three IV and one arterial contrast agents were used: Levovist(R), DMP115, Quantison(TM) and Myomap(TM). Levovist(R) showed the highest sensitivity to oxygen concentration in the solution, and the other three proved tolerant for the above values of oxygen concentrations. (C) 2000 World Federation for Ultrasound in Medicine & Biology.