Biocompatible microspheres containing one or more active principals, a biodegradable and biocompatible polymer and a surface-active agent which is also biodegradable and biocompatible, contain less than 10 ppm of heavy metals. These microspheres are prepared by providing a solution of the polymer and of the active principal in a water-immiscible solvent which is more volatile than water and mixing with an aqueous solution of the surface-active agent, followed by evaporation of the solvent.