The three human anti-plague Fabs were used in competition-ELISAs against a panel of mouse anti-plague mAbs. The mouse anti-plague mAbs included the anti-F1 mAb F1-04-A-G1, and five anti-V mAbs, which included the anti-V mAb 7.3 m that was highly protective. We found no apparent competition between the human anti-V m253 Fab and the mouse anti-V mAbs (Figure 2a ). However, we observed some weak competition between the human anti-V m254 Fab antibody and some of the mouse anti-V mAbs (7.3 m, 10-1 m, and 74-1 m) that we did not see with the human anti-V m253 Fab antibody (Figure 2b ). The competition between the human anti-F1 m252 Fab antibody and the mouse anti-F1 mAb was also minimal (Figure 2c ). We ran two controls in the competition studies with the human and mouse mAbs. For one control we did not add a primary antibody (labeled NC, Figure 2a -2c), and for a second control we used a nonspecific mouse isotype IgG1 mAb in the competition assay (labeled Bm, for a Burkholderia mallei IgG1 mAb). There was also a lack of competition between the human anti-V m253 and m254 Fab antibodies, suggesting that these two human anti-V Fabs recognize different epitopes (one which may be conformational) on the V-antigen (Figure 2d ). Of note, however, is that when the competition-ELISA was performed in a different fashion, namely when the human anti-F1 or anti-V mAbs were allowed to bind to the respective antigens before adding the mouse anti-F1 or anti-V mAbs, moderate competition was detected between the human anti-F1 m252 and the mouse anti-F1 mAbs, as well as between the human anti-V m253 and mouse anti-V 84-1 mAbs (data not shown).