Disclosed are suspensions of gas microbubbles immobilised within a frozen aqueous carrier liquid comprising usual additives and stabilisers, in which the carrier liquid is a physiologically acceptable, the immobilised gas microbubbles are microbubbles bound by an evanescent envelope or a tangible membrane. The suspensions, when in liquid form, are injectable and useful as a contrast agents in ultrasonic imaging of blood pool and tissue of living beings. The gas microbubbles are immobilised within the carrier by freezing a suspension of microbubbles with average sizes below 50 .mu.m, preferably below 10 .mu.m and more preferably between 2 .mu.m and 8 .mu.m, to a temperature between -1.degree. C. and -76.degree. C. and maintaining this temperature for prolonged periods of time. The microbubbles may be stabilised by a surfactant such as a lamellar phospholipid or may comprise a membrane made of synthetic or natural polymer or protein. A method of cold storage of microbubble suspensions as well as their use is also disclosed.