A process for preparing a particle comprising a biological molecule typically a DNA plasmid, and a carrier polymer, typically poly-lactic acid (PLA), in which an organic solvent solution of the biological molecule and the carrier polymer in an organic solvent medium, nearly saturated with the biological molecule, is prepared, then this solution is contacted with an antisolvent substance, typically supercritical carbon dioxide to separate a particle comprising the biological molecule and the carrier polymer. High loadings of the biological molecule in the particle can be achieved.