A hepatocyte-directed vector has been developed; it includes several key features thought to favor in vivo gene delivery to the liver: electrostatically neutral particles which avoid nonspecific binding to other cells, to the extracellular matrix, and to complement proteins; asialoglycoprotein receptor-mediated endocytosis which may address the complexes to the perinuclear region; and polyethylenimine (PEI)-mediated endosome buffering and swelling as an escape mechanism to the cytoplasm. This system is based on a 5% galactose-bearing polyethylenimine (PEI-gal) polymer which is condensed with plasmid DNA to neutrality. Murine (BNL CL.2) and human (HepG2) hepatocyte-derived cell lines were transfected 10(4)-10(5)-fold more efficiently than murine fibroblasts (3T3), whether transfection was assessed globally (luciferase expression from the cell extract) or following histochemical staining (beta-galactosidase). Under these conditions, over 50% of the hepatocytes were selectively transfected in the presence of 10% serum. Transfection was suppressed by removal of the targeting galactose residues, by their replacement with glucose, or by the addition of excess asialofetuin. Thus, results from comparative and competitive experiments indicate the asialoglycoprotein receptor is involved in transfection of hepatocytes with neutral PEI-gal/DNA complexes.