[en] A new gene transfer system was developed by using polylipoplexes, which were prepared by hydrophobic modification of polyethyleneimine (PEI, MW 2000). PEI 25kDa is well known for its excellent transfection efficiency but it has extreme cytotoxicity; therefore, its application for medical use is strictly limited. PEI 2kDa is able to form complexes with DNA and has low cytotoxicity. However, unfortunately, it shows no transfection efficiency so it can not be a candidate carrier for gene therapy. We designed novel polycationic amphilphiles by conjugating hydrophobic moieties, such as cholesterol and myristate, to PEI 2kDa. Cholesterol-conjugated PEI (PEI-Chol: P10C, P17C and P30C) and myristate-conjugated PEI (PEI-Myr:P10M, P16M and P26M) are different from the other cationic lipids in that they can form lipopolyplexes with plasmid DNA that have extra multi-positive charges in their hydrophilic parts. From a different point of view, they are also considered to be PEI derivatives with a small proportion of hydrophobic moiety. As a result of the modification, PEI-Chol and PEI-Myr showed much enhanced transfection activity but somewhat increased cytotoxicity. We also examined the effect of the amount of hydrophobic moiety on lipopolyplex-mediated gene transfer and observed that P17C and P26M are the most effective carriers in the series of two groups. MTT assay indicated that the more myristyl groups were attached to PEI, the more injurious results were observed. In the case of PEI-Chol, however, the opposite tendency was observed