Gene transfection mediated by the cationic polymer polyethylenimine (PEI) is considered a standard methodology. However, while highly branched PEIs form smaller polyplexes with DNA that exhibit high transfection efficiencies, they have significant cell toxicity. Conversely, low molecular weight PEIs (LMW-PEIs) with favourable cytotoxicity profiles display minimum transfection activities as a result of inadequate DNA complexation and protection. To solve this paradox, a novel polyelectrolyte complex were prepared by the ionic cross-linking of branched 1.8kDa-PEI with citric acid (CA). This system exploits synergistically the good cytotoxicity profile exhibited by LMW-PEI with the high transfection efficiencies showed by highly branched and high molecular weight PEIs. The polyectrolyte complex (1.8kDa-PEI@CA) was obtained by a simple synthetic protocol based on the microwave irradiation of a solution of 1.8kDa-PEI and CA. Upon complexation with DNA, intrinsic properties of the resulting particles (size and surface charge) were measured and their ability to form stable polyplexes was determined. Compared with unmodified PEIs the new complexes behave as efficient gene vectors and showed enhanced DNA binding capability associated with facilitated intracellular DNA release and enhanced DNA protection from endonuclease degradation. In addition, while transfection values for LMW-PEIs are almost null, transfection efficiencies of the new reagent range from 2.5 to 3.8 fold to those of LipofectamineTM 2000 and 25kDa-PEI in several cell lines in culture such as CHO-k1, FTO2B hepatomas, L6 myoblast or NRK cells, showing simultaneously a negligible toxicity. Furthermore, the 1.8kDa-PEI@CA polyelectrolyte complexes retained the capability to transfect eukaryotic cell in the presence of serum and exhibited the capability to promote "in vivo" transfection in mouse (as an animal model) with an enhanced efficiency compared to 25kDa-PEI. Results support polyelectrolyte complex of LMW-PEI and CA as promising generic non-viral gene carriers.