Lactosylated poly-l-lysine is a nonviral vector that transfers genes into airway epithelial cells, including those from individuals with cystic fibrosis (CF). Substitution of 40% of the ε-amino groups of poly-l-lysine with lactosyl residues not only provided a ligand for receptor-mediated endocytosis, but also reduced the toxicity when compared with nonsubstituted poly-l-lysine. Lactosylated poly-l-lysine/pCMVLuc complex is not toxic to cells in amounts that gave the maximum gene expression. The level of gene expression was regulated by using different combinations of chloroquine, glycerol, and E5CA peptide. Using cultured CF cells, chloroquine, combined with E5CA peptide, increased the transfer of the pCMVLuc/ lactosylated poly-l-lysine complex 10,000-fold compared with transfer without additives. In many systems, a high efficiency is of paramount importance and the enhancing agents can be used to modulate the expression of the gene. For example, transfer of pCMVLacZ/lactosylated poly-l-lysine complexes with chloroquine added to the transfection medium gave only 20% transfection efficiency of the reporter gene. However, when chloroquine was combined with glycerol, the efficiency was increased to 90%, thus approaching that reported with viral vectors. This highly efficient vector may be of great value for the future development of gene transfer systems.