To evaluate the potentiality of applying gene therapy to bacterial infections, especially for preventing infection in high-risk patients, we investigated protection of mice from challenge with lethal Escherichia coli infection by adeno-associated virus serotype 2 (AAV2)-mediated gene transfer of a chimeric BPI23-Fcgamma1 gene, which consisted of human bactericidal/permeability-increasing protein (BPI) gene encoding the functional N terminus (amino acid residues 1 to 199) of human BPI and an Fcgamma1 gene encoding the Fc segment of human immunoglobulin G1. Here we show that the target protein that was expressed and secreted into the serum of the gene-transferred mice demonstrated the activity of a neutralizing endotoxin, killing E. coli and mediating opsonization. After lethal E. coli infection, the count of bacteria and the levels of endotoxin and proinflammatory cytokines in the gene-transferred mice were decreased. The survival rate of BPI23-Fcgamma1 gene-transferred mice markedly increased, especially in conjunction with antibiotics. Our data suggest that AAV2-mediated chimeric BPI23-Fcgamma1 gene delivery could potentially be used clinically for the protection and treatment of infection with gram-negative bacteria in high-risk individuals.