The consequences of macrophage depletion achieved by intravenous infusion of liposome-encapsulated clodronate (dichlormethylene diphosphonate (Cl2MDP)) on adenovirus-mediated transfer of a recombinant human alpha 1-antitrypsin (hAAT) gene were examined in 12-14-week-old male Balb/c mice. The levels of hAAT expression following tail vein infusions of 10(9) p.f.u. of Ad.RSV-hAAT were approximately four-fold higher in macrophage-depleted animals than in control animals pretreated with liposome-encapsulated phosphate-buffered saline (PBS). Clodronate pretreatment also significantly increased the survival of animals injected with high doses of viral vector. Long-term studies performed in animals receiving tail vein infusions of the adenoviral vector also indicated that clodronate pretreatment significantly attenuated the rapid loss of transgene expression usually observed in immunocompetent animals. These findings indicate that the depletion of macrophages before adenovirus-mediated gene transfer may increase the transduction efficiency and reduce the rate of immunologic elimination of the adenovirally transduced cells, thereby increasing the persistence of transgene expression in immunocompetent animals.