The phenomenon of enhanced backscatter from myocardial contrast agents was studied using two examples, a robust thicker-walled, intra-arterial agent (AIP 201) and a smaller thinner-walled, intravenous agent (Quantison(TM)). Both agents are composed of albumin-encapsulated microbubbles. Samples of the agents were inserted into an in vitro phantom and insonated under different scanning regimes. Upon insonation, Quantison(TM) exhibited a pronounced increase in mean backscatter at medium and low concentrations, which decreased dramatically with increasing number of frames of insonation. At high concentrations, no dramatic decrease or increase in mean backscatter was observed over the period of the experiment. AIP 201 exhibited an overall decrease in mean backscatter with increasing number of frames of insonation. These results suggest that the difference in size and wall thickness of the contrast microcapsules can significantly affect the behaviour of the contrast agents in an ultrasound held. (C) 1998 World Federation for Ultrasound in Medicine & Biology.