To improve the biocompatibility of a gene vector and to avoid its being eliminated by the immune system, polyethylenimine (PEI) was modified with poly(ethylene glycol) (PEG) before G250 monoclonal antibody (mAb) conjugation. G250-PEI-PEG was capable of forming complexes with DNA in the right size distribution, and the G250 mAb modification significantly improved PEI transfection of G250-positive cells. The highest transfection efficiency was seen in HeLa cells as determined by flow cytometry after transfection with the gene encoding green fluorescent protein: 2-fold higher compared with the transfection of HepG2 cells. Blocking the surface antigen on the cell membrane of HeLa cells by incubation with free G250 mAb, or by downregulating G250 expression by small interfering RNA transfection, resulted in a remarkable decrease in transfection efficiency. These data indicate the targeting effect of G250 antibody modification. The presence of serum decreased transfection efficiency in a concentration-dependent manner. However, the transfection of HeLa cells with G250-PEI-PEG remained significant in the presence of 30% serum. In an in vivo study, G250-PEI-PEG exhibited high transfection efficiency in tumors. In addition, pathological analysis did not show obvious toxicity caused by the materials used. These suggest that PEG- and G250 mAb-modified PEI could be a useful nonviral gene vector for in vivo study.