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We have identified six monoclonal antibodies (MAbs) mapping to both linear and conformation-dependent epitopes within the V2 region of the human immunodeficiency virus type 1 clone HXB10. Three of the MAbs (12b, 66c, and 66a) were able to neutralize the molecular clones HXB10 and HXB2, with titers in the range of 9.5 to 20.0 micrograms/ml. MAbs mapping to the crown of the V2 loop (12b, 60b, and 74) bound poorly to cell surface-expressed oligomeric gp120, suggesting an explanation for the poor or negligible neutralizing activity of MAbs to this region. In contrast, MAbs 12b and 60b demonstrated good reactivity with recombinant gp120 in an enzyme-linked immunosorbent assay format, suggesting differential epitope exposure between the recombinant and native forms of gp120. Cross-competition analysis of these MAbs and additional V1V2 MAbs for gp120 binding enabled us to assign the MAbs to six groups (A to F). Selection of neutralization escape mutants with MAbs 10/76b and 11/68b, belonging to nonoverlapping competition groups, identified amino acid changes at residues 165 (I to T) and 185 (D to N), respectively. Interestingly, these escape variants remained sensitive to neutralization by the nonselecting V2 MAbs. All MAbs demonstrated good recognition of IIIB viral gp120 yet failed to neutralize nonclonal stocks of IIIB. In addition, MAbs 12b and 62c bound MN and RF viral gp120, respectively, yet failed to neutralize the respective isolates. Cloning and expression of a library of gp120 and V1V2 fragments from IIIB-, MN-, and RF-infected H9 cultures identified a number of polymorphic sites, resulting in antigenic variation and subsequent loss of V2 MAb recognition. In contrast, the V3 region from the clones of the same isolates showed no amino acid changes, suggesting that the V2 region is polymorphic in long-term-passaged laboratory isolates and may account for the reduced antibody recognition observed.