We recently reported that translocating murine polyreactive anti-DNA antibodies can be used as vectors for the transfer of macromolecules into cells growing in culture. We show here that two such monoclonal antibodies (J20.8 and F4.1) conjugated to polylysine with a high (93) but not a low (19) number of lysine residues can transfer genes in the presence of serum. A 30 amino acid long peptide, VAYISRGGVSTYYSDTVKGRFTRQKYNKRA (peptide P3), corresponding to joined heavy-chain complementary-determining regions 2 and 3 of F4.1 antibody and carrying 19 lysine residues at its N-terminal, was found to be an efficient vector for the transfection of the luciferase gene into 3T3 and CCL39 cells in the presence of serum. Addition of 0.23 M glycerol during transfection considerably enhanced gene delivery. These results show that conjugation of a short polylysine tail converted a spontaneously internalizing peptide into a potent nontoxic plasmid vector.