We previously developed complexes of lipoplexes containing 3beta-(N-(N',N'-dimethylaminoethane)carbamoyl)cholesterol (DC-chol) and succinylated poly(glycidol)-modified liposome, which becomes fusogenic under weakly acidic condition, for use as a novel gene delivery system. This study explored the effect of lipoplex structures--the type of cationic lipid and cationic lipid/DNA charge ratio--on the transfection activity of those complexes. Three types of cationic lipid with different polar groups were used for the preparation of lipoplexes: DC-chol, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate (DOTAP), and 3,5-dipentadecyloxybenzamidine (TRX-20) with dimethylamino group, trimethylammonium group, and benzamidine group, respectively. Complexation with the SucPG-modified transferrin-bearing liposomes affected transfection activity of these lipoplexes differently. The TRX-20 lipoplexes exhibited the most marked enhancement of transfection activity upon complexation with the SucPG-modified liposomes among these lipoplexes. The cationic lipid/DNA charge ratio of the lipoplex and the amount of the transferrin-bearing SucPG-modified liposomes associated to the lipoplex also affected the transfection activity of the resultant complexes. Highly potent gene vectors were obtained by adjusting these factors.