To enhance the transfection efficiency of the cationic liposomes, chitosan was selected as a DNA condensing agent. The particle size of the liposome/chitosan/DNA ternary complexes decreased and the zeta potential increased with the addition of chitosan. The formation of the ternary complexes was identified using agarose gel retardation study. The interaction of the ternary complexes was further confirmed by the decrease of the DNA fluorescence in the presence of [Ru(phen)2dppz](2+). In vitro and in vivo transfection activities of the complexes were determined using green fluorescent protein (GFP) expression in various cell lines and mouse tibial anterior muscle subcutaneously, respectively. Liposome/chitosan/DNA ternary complexes showed improved transfection efficiency in vitro cell culture system in the presence or absence of serum as well as in vivo mouse model system, as compared with liposome/DNA lipoplex. More importantly, the cell toxicity of the ternary complex is lower than that of lipoplex and liposome/poly-L-lysine/DNA ternary complex. The precondensation of DNA with chitosan can be a promising approach to further increase the transfection efficiency of cationic liposomes in clinical application. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.