During HIV-1 infection, the current tenet is that only anti-gag antibodies decline, while those directed at env remain stable. Among the latter antibodies, those directed to the V3 domain of gp120 are assumed to play a role in the immune surveillance against HIV-1. We investigated the correlation between anti-V3 antibody levels and the clinical stage of infection and the ability to neutralize syncytium formation. Using a V3-specific antigen-limited ELISA, we analysed the antibody levels of a panel of 93 HIV-1+ sera to V3 peptides derived from different HIV-1 strains and from the North American/European consensus sequence V3(Cs). Sera preferentially recognized V3 peptides from the representative V3(MN) strain and V3(Cs). Antibody reactivity to V3(MN) or V3(Cs) actually declined in relation with progression to AIDS, while antibodies against whole recombinant gp160 or gp41 immunodominant epitope remained stable. There was a strong correlation (P < 0.0001) between anti-V3 (Cs)/V3(MN) antibody levels and serum titres that neutralized HIV-1MN-mediated syncytia. Serology based on V3-specific antigen-limited ELISA indicates that anti-V3 antibody reactivity may decline during the course of HIV infection.