Monosized cationic nanoparticles were produced by emulsifier-free emulsion polymerization of styrene, poly(ethylene glycol) methacrylate and N-[3-(dimethyl-amino) propyl] methacrylamide, conducted in the presence of a cationic initiator, 2,2'-azobis (2-methylpropionamidine) dihydrochloride, using different amounts of ingredients and at different conditions. The structure of the terpolymers was confirmed by H-1-NMR and FTIR spectroscopy. The nanoparticles, with an average size of 71.3 nm [polydispersity index (PDI), 1.110] and a Zeta potential of 65.6 mV obtained by a zeta sizer, were used in the transfection studies. HeLa cells were transfected in in vitro cell cultures with these non-viral nanoparticle vectors with a green fluorescent protein (GFP)-expressing plasmid. DNA. The transfer of the cationic nanoparticles into the cells and GFP expressions with the conjugates of the nanoparticles and the GFP-expressing plasmid were followed by both light and fluorescent microscopy. The GFP expression efficiency was unexpectedly high (up to 90%). Copyright (C) 2008 John Wiley & Sons, Ltd.