To examine the potential use of adenovirus vectors in cancer gene therapy as a mechanism for purging bone marrow cells of possible breast cancer contaminants, we compared the infection efficiency of adenovirus and the transfection efficiency of plasmid DNA in the presence of adenovirus in human breast cancer and bone marrow cells. Following infection of breast cancer cells with an adenovirus expressing beta-galactosidase gene, high levels of beta-galactoside activity were observed. No beta-galactosidase activity was observed in low-density human bone marrow cells. A replication-deficient adenovirus mutant dl312 enhanced the transfection efficiency of a plasmid DNA-expressing beta-galactosidase gene into breast cancer cells, and addition of a liposome, lipofectamine, further enhanced the transfection efficiency. In contrast, human bone marrow cells treated under the same conditions expressed very low levels of transfected beta-galactosidase DNA. Transfection of cells with plasmid DNA expressing a truncated but fully active Pseudomonas exotoxin gene in the presence of dl312 and lipofectamine resulted in marked breast cancer cell killing, whereas colony-forming unit granulocyte-macrophage (CFU-GM) were relatively resistant to these treatments. A recombinant adenovirus expressing human wild-type p53 protein (AdWTp53) was also highly cytotoxic to breast tumor cells. Infection of breast cancer cells with AdWTp53 (100 plaque-forming units/cell) resulted in 100% loss of the clonogenicity of breast tumor cells. However, colony formation from CFU-GM was relatively resistant to the cytotoxic effects of AdWTp53 alone or in the presence of pULI100 plasmid and lipofectamine. On the basis of these results, it is proposed that human adenoviruses are potentially useful for cancer gene therapy and bone marrow purging.