A conserved N-glycan present within the V3 loop of gp120 modulates the sensitivity to neutralization by antibodies directed to the V3 loop. A glycan-deficient mutant of HIVLAI, designated HIVA308, displayed a 100-fold increase in sensitivity to neutralization by anti-V3 MAb NEA-9205 compared to wild-type HIVLAI. This difference in sensitivity was not caused by an alteration of the antibody binding site itself, as NEA-9205 had equal affinity for both wild-type and mutant monomeric gp120. In contrast, virion-associated wild-type gp120 was immunoprecipitated less efficiently with NEA-9205 than virion-associated mutant gp120. This difference was completely abrogated, if immunoprecipitation were carried out in the presence of detergent. Furthermore, treatment of virion preparations with detergent exposed the C-terminal D7324 epitope, which is inaccessible on virion-associated gp120 but readily accessible on monomeric, soluble gp120. Finally, both wild-type and mutant monomeric, soluble gp120 were precipitated equally efficiently by NEA-9205 in the absence of detergent. Thus, the NEA-9205 epitope was readily accessible on monomeric gp120 regardless of the presence of the 306N-glycan, and inaccessibility of the NEA-9205 epitope imparted by the 306N-glycan was observed only on the intact envelope oligomer.