Protein was precipitated from organic and aqueous solutions using carbon dioxide and ammonia as antisolvents. The gas antisolvent precipitation process (GAS) was used to produce lysozyme, insulin and myoglobin powders. Protein powders were produced with narrow size ranges, and particle size was controlled between 0.05 mu m and 2.0 mu m by changes to the solvent system. Typically the stronger the protein solvent the larger the precipitate size. The GAS process, although ideal for the micronisation of stable protein powders, was limited by the number of suitable protein solvents that were miscible with dense carbon dioxide and that did not irreversibly affect protein conformation. As a result, GAS precipitation from aqueous solutions was also assessed. Insulin was precipitated from aqueous solutions as discrete 0.2-0.3 mu m spheres using ammonia as an antisolvent.