Development of neutralizing antibodies is an important hindrance that limits repeated administration of adenoviral vectors for gene transfer. One way to avoid this problem would be to coat the virus with a substance that could shield it from antibodies. To develop such a system, we coated negatively-charged adenovirus with the cationic lipid GL-67 and included polyethylene glycol (PEG) in the complex as dioleoylphosphatidylethanolamine-PEG (DOPE-PEG). This complex enhanced gene transfer to cells that were difficult to infect both in vitro and in vivo. GL-67/DOPE-PEG coated the virus and prevented antibody binding. As a result, 50-fold higher concentrations of immune plasma were required for neutralization than with adenovirus alone. However, use of the complex provided no appreciable protection from neutralization when vector was delivered in vivo to immunized animals. These data are the first to suggest that formation of a complex around adenovirus can partially shield it from immune plasma in vitro. Despite the lack of protection in vivo, these results suggest the feasibility of developing a system in which the virus is effectively shielded from neutralizing antibodies and capable of repeat administration.