V antigen of Yersinia pestis is a multifunctional protein that has been implicated as a protective antigen, a virulence factor, and a regulatory protein. A series of V-antigen truncates expressed as glutathione S-transferase (GST) fusion proteins (GST-V truncates) have been cloned and purified to support immunogenicity and functionality studies of V antigen. Immunization studies with GST-V truncates have identified two regions of V antigen that confer protection against Y. pestis 9B (a fully virulent human pneumonic plague isolate) in a mouse model for plague. A minor protective region is located from amino acids 2 to 135 (region I), and a major protective region is found between amino acids 135 and 275 (region II). In addition, analysis of IgG titers following immunization suggested that the major antigenic region of V antigen is located between amino acids 135 and 245. A panel of monoclonal antibodies raised against recombinant V antigen was characterized by Western blotting against GST-V truncates, and epitopes of most of the monoclonal antibodies were mapped to region I or II. Monoclonal antibody 7.3, which recognizes an epitope in region II, passively protected mice against challenge with 12 median lethal doses of Y. pestis GB, indicating that region II encodes a protective epitope. This is the first report of a V-antigen-specific monoclonal antibody that will protect mice against a fully virulent strain of Y. pestis. The combined approach of passive and active immunization has therefore confirmed the importance of the central region of the protein for protection and also identified a previously unknown protective region at the N terminus of V antigen.