We report on the technical advance of linearized pDNA (pDNA(linear)) above the circular one (pDNA(circ)) for preparation of small-sized DNA/polycation complexes (DPC) at a low pH. Also, the resistance of the DPC formed with pDNA(linear) against poly-L-asparagine indicates that effective ion-pairing occurred between the pDNA(linear) and polycations. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.