Several viral and nonviral methods have introduced functional genes into the lungs. An alternative strategy, receptor-mediated gene transfer, exploits the ability of receptors on the surface of cells to bind and internalize DNA complexes and could potentially be used to deliver genes to specific cells in the lung. The gene encoding human alpha1-antitrypsin (A1AT) was delivered to macrophages in vitro and in vivo by targeting the mannose receptor with mannose-terminal molecular conjugates. The human A1AT transcript was detected 2 d after transfection of macrophages in culture, but transgene expression was transient. Human A1AT protein was secreted into the culture medium, and Western blot hybridization revealed the mature human antiprotease. In addition, Sprague-Dawley rats underwent intravenous injections of increasing doses of plasmid DNA (0.2 mg, 1.0 mg, and 2.0 mg) complexed to the molecular conjugate. Four days after transfection, human A1AT mRNA was found in lungs from six of the 13 rats (46%) that received the higher doses of plasmid. Transgene expression was limited to cells in perivascular and alveolar regions, which conformed to the distribution of pulmonary macrophages. Human A1AT was measured in the epithelial lining fluid of rats treated with transfection complexes. Animals that received 1.0 mg of plasmid had human A1AT levels of 7.4 +/- 3.4 pM, which was significantly different from nontransfected and mock-transfected controls. Thus the mannose receptor permitted direct delivery of genes to pulmonary macrophages, though transgene expression was detected in the lung only at low levels.