The anti-F1 mouse mAb F1-04-A-G1 (or mF1) was provided by George Anderson (USAMRIID) [13] , and anti-V mouse mAbs 10-1 m, 74-1 m, 84-1 m, and 141-1 m as well as the control IgG1 mouse anti-Burkholderia mallei (Bm) antibody were obtained from Sylvia Trevino (USAMRIID) and anti-V mouse mAb 7.3 (7.3 m) was obtained from Jim Hill (Porton Down, Wiltshire, UK) [14] and used in competition ELISAs and as positive controls in mouse passive protection experiments. All mice mAbs were IgG1 isotypes. The human IgG1 control mAb used in the passive protection study was an anti-human IGFII mAb.