Efficient DNA delivery is a prerequisite for the successful implementation of molecular antiviral strategies against chronic viral hepatitis and gene therapy in general. The cationic polymer polyethylenimine (PEI) has recently been explored as a gene transfer vector in various cell types in vitro and in vivo. In this study, we evaluated a linear PEI derivative (lPEI) as a vector for gene and oligodeoxynucleotide transfer into hepatocytes in vitro and in vivo. A simple protocol was developed that allowed transfection of up to 50% of primary hepatocytes in vitro. In addition, fluorescent oligodeoxynucleotides were efficiently delivered to the liver in vivo after intravenous injection into Pekin ducks. Thus, lPEI mediates highly efficient gene and oligodeoxynucleotide transfer into primary hepatocytes and is potentially useful for DNA delivery in vivo.