We have previously developed the succinylated poly(glycidol)-modified liposome which becomes fusigenic under weakly acidic condition. In this report, we describe that complexation of this pH-sensitive, fusigenic liposome with a lipoplex consisting of 3␤-(N-(NЈ,NЈ-dimethylaminoethane) carbamoyl)cholesterol, dioleoylphosphatidylethanolamine and plasmid DNA gives efficient gene delivery systems. In this study, we prepared the complexes, which are termed SucPG-complexes, with a positively or negatively charged surface by mixing the lipoplex with varying amounts of the SucPG-modified liposomes. The positively charged SucPGcomplexes either bearing or not bearing a cell-specific