The binding of Abs to HIV-1 derived peptides or proteins does not necessarily mean that these mAbs will be able to bind intact viruses, which essentially present a more native conformation. In order to address this possibility, we tested the anti-V3 mAbs for binding with Du156.12 (subtype-C) and JRFL (subtype-B) viruses in an intact virion binding assay. The two viruses (Du156.12 and JRFL) were chosen for this experiment on the basis of their V3 sequence similarity with the corresponding con-C and B V3 sequence ( Figure 2C ). Consistent with the binding of anti-V3 mAbs to Du156.12 and JRFL derived envelope gp120 proteins, the three mAbs showed differential binding affinity to intact virions, 904 displaying high affinity as compared with 277 and 903. In addition, the mAb 904 retained the crossreactive binding potential to the intact viruses, evident from the binding pattern ( Figure 2A ). The experiment was validated by testing the intact virion binding of anti-V3 mAb 447-52D to SF162 virus, known to have a well exposed V3 region and allows accessibility to Ab without any interference [43] [44] [45] . The binding of 447-52D to SF162 intact virus revealed a very high binding affinity (more than five folds) of mAb 447-52D as compared to binding of three anti-V3 mAbs to Du156.12 and JRFL at equivalent viral concentration (i,e 25 ng/ml) (Figure 2A -B). This differential binding of mAbs to intact virions could also be attributed to the number of potential N- Table 3 Binding of mAbs to subtype-A, B and C derived HIV-1 proteins and peptides 2 anti-V3 mAbs 1 Protein/peptide Subtype I 277 I 903 I 904 A 447-52D 1418