HZQ and JES conceived of the study and wrote the first draft of the manuscript, HZQ, HTC and YHR collected the data, and all authors participated in the data interpretation and manuscript revisions. Development of a humanized monoclonal antibody with therapeutic potential against West Nile virus Neutralization of West Nile virus (WNV) in vivo correlates with the development of an antibody response against the viral envelope (E) protein. Using random mutagenesis and yeast surface display, we defined individual contact residues of 14 newly generated monoclonal antibodies against domain III of the WNV E protein. Monoclonal antibodies that strongly neutralized WNV localized to a surface patch on the lateral face of domain III. Convalescent antibodies from individuals who had recovered from WNV infection also detected this epitope. One monoclonal antibody, E16, neutralized 10 different strains in vitro, and showed therapeutic efficacy in mice, even when administered as a single dose 5 d after infection. A humanized version of E16 was generated that retained antigen specificity, avidity and neutralizing activity. In postexposure therapeutic trials in mice, a single dose of humanized E16 protected mice against WNV-induced mortality, and may therefore be a viable treatment option against WNV infection in humans. SUPPLEMENTARY INFORMATION: The online version of this article (doi:10.1038/nm1240) contains supplementary material, which is available to authorized users. Development of a humanized monoclonal antibody with therapeutic potential against West Nile virus E protein by nickel-affinity chromatography (data not shown). After immunization and screening 2,000 hybridomas, we isolated 46 new monoclonal antibodies that recognized WNV E protein (Supplementary Table 1 online).