Abstract: AimTo avoid the limitation of the use of cationic polyethlenimine (PEI)-complexed plasmid DNA use for in vitro or in vivo gene delivery due to its cytotoxicity and lower efficiency in the presence of serum. MethodsA polyplex with decreased positive charge on the complex surface was designed. The PEI/DNA (PD) complexes coated with an anionic biodegradable polymer, alginate were prepared and their gene delivery behavior with PD was compared. ResultsThe alginate-coated PD polyplex, where alginate:PEI:DNA ［alginate:DNA, 0.15(w/w); PEI:DNA, N:P=10］ showed about 10-30 fold-increased transfection efficiency compared to corresponding non-coated complexes to C3 cells in the presence of 50% serum. The surface charge of the alginate-coated complex was approximately half of that of the alginate-lacking complex. The size of alginate-coated complex was slightly smaller than that of the corresponding complex without alginate. The former complex also showed a reduced erythrocyte aggregation activity and decreased cytotoxicities to C3 cells in comparison with PD complex. ConclusionThe alginate-coated PD polyplexes as a new gene delivery system can improve transfection efficiency in high serum concentration with low cytotoxicity to C3 cells.