To characterize the binding of the human anti-F1 and anti-V mAbs to the F1-and V-antigens, respectively, more closely, we examined the binding of the human mAbs to two separate panels of overlapping peptides. One panel covered the full-length of the F1 antigen (27 peptides) and the other -the V antigen (53 peptides). For the human anti-F1 m252 mAb, there was a weakmoderate binding signal with peptides 1 and 2, which are located at the N-terminus of the F1-antigen (Figure 3a ). This suggests that the m252 mAb may also recognize a conformational region that involves peptides 1 and 2. The binding by human anti-V m253 mAb resulted in a strong signal with peptide 2 and a weak signal with peptide 1 (Figure 3b ). The anti-V m254 mAb, did not bind to any of the peptides, suggesting that its epitope may be conformational ( Figure 3c ). In initial studies with m254 we saw some weak binding to peptides 36 and 42, but when we repeated the binding studies with the human anti-V mAbs we did not see binding to peptides 36 and 42. This might explain its weak competition with mouse antibodies that recognize diverse epitopes on the V-antigen ( Figure 2b ). The positive signals seen with the m253 and m254 mAbs with V-antigen peptides (numbers 19, 20, 27, and 28) are nonspecific signals that are generated by the secondary antibody (Amemiya et al. unpublished). The epitopes of the mouse antibodies have also been determined by the peptide binding assay (Amemiya, et al. unpublished). The data is consistent with the competition-ELISA presented in this study.