Air-filled polymeric microcapsules for use as a contrast agent in ultrasonography have been prepared by the freeze-drying of different oil-in-water emulsions. The water phases consisted of a block copolymer in water. The organic phases consisted of a biodegradable polyester dissolved in (-)-camphene, cyclooctane, cyclohexane or tricyclene, which were relatively poor solvents for the polyester. A polymeric wall was, therefore, precipitated at the droplet surface early in the process, i.e. during freezing. Removing the solvent during freeze-drying, resulted in air-filled microcapsules. The microcapsules were suspended in saline after freeze-drying. All the suspensions contained echogenic microcapsules with a volume mean diameter of similar to5-7 mum. Microscopic investigations showed that the microcapsules were spherical and hollow. Tricyclene and, to some degree, (-)- camphene were found unsuitable for industrial production due to melting points above 30 degreesC. Cyclooctane and cyclohexane were investigated as replacements for the initially chosen (-)-camphene, since they are liquids over a wider temperature range. These solvents gave improved yields, measured both as particle volume concentration per amount of polymer in suspension and acoustic attenuation at 3.5 MHz per amount of polymer in suspension, although the freeze-drying cycle was not optimized for these systems.