ONE approach to the controlled release of drugs involves incorporation of the drug molecules into the matrix of microscopic polymer spheres or capsules1-10. Existing methods for preparing such microparticles do not, however, always guarantee a constant release rate, for example because drug molecules may be trapped preferentially at the surface, because they have to diffuse through an increasing thickness of polymer when the particles are non-eroding or because the surface area changes for eroding particles. In other situations pulsed release may be required-an application to which simple polymer microspheres do not readily lend themselves. Multi-walled microspheres might solve some of these problems. Here we describe a one-step process for preparing double-walled polymer microspheres with diameters ranging from about 20 to 1,000 micrometres. Our technique11 involves the phase separation of a polymer mixture owing to solvent evaporation : with an appropriate choice of interfacial tensions and evaporation rate, a spherical droplet of one polymer becomes coated with a highly uniform layer of the other. This process, which might be adapted to yield multi-walled microspheres, should make possible the engineering of highly specific drug-release properties.