Water-soluble PEG derivatives having both amino- and carboxyl-pendants (PEG-ACs) were synthesized, and examined for their transcription- and transfection-enhancing activity on DNA/polycation complexes. PEG-AC could be deposited onto the surface of DNA/polyethylenimine(PEI) complexes, and enhanced their transcriptional activity. Fluorescence anisotropy study showed that the amphoteric PEG-AC loosened the tightly compacted DNA/PEI complex to facilitate the approach of transcriptional factors. Transcriptional activity of the complex was in good correlation with its transgene expression efficiency, indicating the importance of improvement in transcription for effective gene transfection.