Microbubble dispersions stabilised by phospholipids predominantly comprising molecules which individually have an overall net charge exhibit advantageous stability, rendering them useful as efficacious contrast agents. An improved process for preparing microbubble-containing contrast agents is also disclosed, this comprising lyophilising an aqueous dispersion of gas microbubbles stabilised by one or more membrane-forming lipids to yield a dried product which may be reconstituted in an injectable carrier liquid to generate a microbubble-containing contrast agent.
