The present study demonstrates that the human adenovirus (Ad) can augment transfer and expression of a gene within plasmid DNA unmodified by nonspecific linkers or by linker-ligand complexes. Following the transfection of COS-7 cells with pRSVL, a luciferase expression plasmid vector directed by the Rous sarcoma virus-long terminal repeat promoter, luciferase activity in the target cells was 10(3)- to 10(4)-fold higher when the cells were also infected with Ad-CFTR, a replication-deficient recombinant Ad containing human cystic fibrosis transmembrane conductance regulator (CFTR) cDNA. The enhancement of luciferase gene expression in COS-7 cells was also observed with Ad-dl312 (a replication-deficient E1a deletion mutant Ad with no exogenous gene) and wild type Ad5. The efficiency of cell transfection with pRSVL in the presence of an Ad was achieved in a dose-dependent fashion with progressively higher luciferase activity in cells infected by increasing amounts of Ad-CFTR, Ad-dl312, or Ad5. The augmentation by Ad-CFTR of the transfer and expression of the luciferase gene in cells was similar to that of another transfection reagent, cationic liposomes. Further, when Ad-CFTR and liposomes were used in combination, 4- to 100-fold more efficient expression of the luciferase gene was achieved than with Ad-CFTR or liposomes alone. When COS-7, HeLa, and CV-1 cells were evaluated in parallel in the presence or absence of liposomes, Ad-mediated enhancement of luciferase activity was observed in all cell lines. Thus, exposure of target cells to replication-deficient or competent human Ad will markedly augment transfer and expression of the genes within plasmid DNA in mammalian cells in vitro without modifying the plasmid with linkers or linker-ligand complexes, a strategy that should be useful for in vitro and in vivo gene transfer applications.