To test if the human anti-F1 and anti-V IgGs bind their respective targets on bacterial cells, we performed flow cytometry analysis. Both the mouse anti-F1 F1m mAb and human anti-F1 m252 mAb bound specifically to Y. pestis grown at 37uC and not to the same strain grown at 26uC. Neither did they bind to a control E. coli strain grown at 37uC (Figure 4 , bottom panel). This is consistent with previous reports that the expression of the F1-antigen is regulated by temperature (37uC), and it is not expressed at RT. The human anti-V m254 mAb showed minor binding to the Y. pestis grown at 37uC, although we do not normally see binding with the human anti-V m253 or mouse anti-V mAbs, which included 7.3 m, 74-1 m, and 141-1 m (Amemiya, unpublished). A mouse IgG1 isotype control mAb, which did not show any binding to the wholecells, was included to show binding by the mouse and human anti-F1 mAbs was antibody specific. To further confirm the binding data we used immunofluorescence technique. The results were consistent with the flow cytometry data, where only the mouse and human anti-F1 mAb bound to Y. pestis whole-cells ( Figure 5 ). Neither the human anti-V m254 mAb ( Figure 5D ) nor mouse anti-V mAbs, like 7.3 m (data not shown) nor control samples ( Figure 5A , nonspecific mouse IgG1 isotype; Figure 5E , no primary mAb) showed significant binding to Y. pestis.