A short peptide corresponding to the nuclear localization signal (NLS) of human immunodeficiency virus (HIV)-l Tat protein, Arg-Lys-Lys-Arg-Arg-Gln-Arg-Arg-Arg, was employed to improve the efficiency of cellular uptake of nucleic acids. The peptide was first mixed with a reporter plasmid and then with cationic liposomes to form a tripartite complex of DNA/peptide/liposomes (DPL). Transfection efficiency of the DPL complex was compared with that of the conventional DNA/liposomes (DL) complex. When the DPL complex was formed with various cationic liposomes, DOTAP/DOPE (DP) liposome exhibited superior transfection efficiency to other liposomes tested in vitro. With the inclusion of the peptide, the DPL complex showed much enhanced transfection in various cancer cell lines. Particularly, transfection of the DPL complex in serum increased cellular uptake of a transgene up to 2 fold when compared with that in a serum free condition. Further, when the DPL complex was infused through the ureteric route of a rat, transfection efficiency was shown to be better in reporter gene expression than that obtained with the DL complex. This study shows that the DPL complex that is easy to formulate can be employed for much enhanced cellular uptake of a trans gene.