PEI-grafted chitosan (PEI-CMCS) copolymer was synthesized through EDC-mediated amidation reaction between carboxymethyl chitosan and low-molecular-weight polyethyleneimine (LMW PEI). PEI-CMCS and CS/DNA complexes were characterized. Compared with pristine chitosan, the PEI-CMCS exhibited an enhanced ability to condense DNA. Incorporation of LMW PEI to chitosan was found to achieve higher transfection efficiency and much lower cytotoxicity than PEI25K at an optimum weight ratio of 15 in COS-7 cells. However, PEI-CMCS was incompetent to transfer refractory pulmonary epithelial cells. It was demonstrated that doping complex medium or treating cells with a steroid hormone-beta-estradiol significantly improved the transfection efficiency of 16HBE and A549 cells. This study suggests that steroid hormone may become an additive for other cationic polymers to facilitate gene transfection in pulmonary cells. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 874-882, 2011