Poly(ethylene glycol) (PEG)-covered hybrid particles (see Figure) of calcium phosphate (CaP), and plasmid DNA (pDNA) have been prepared in a size-controllable manner in the presence of a block copolymer. The particles show a substantial colloidal stability as well as tolerance of entrapped pDNA against nuclease attack in the medium, and achieve a significantly higher gene expression compared to conventional precipitates.