SummaryHuman immunodeficiency virus type 1 (HIV-1) gp-120 potentially plays an important role in inducing functional suppression and depletion of CD4 lymphocytes following infection with HIV. In order to further understand the mechanisms involved in HIV-induced immunosuppression, we have studied the effects of recombinant HIV-1 gp120/SF2 and anti-gp120/SF2 antibodies on T cell receptor (TCR)-mediated proliferation of peripheral blood mononuclear cells (PBMCs) and isolated lymphocyte subsets from HIV-seronegative donors. In a dose-dependent manner, gp120 significantly reduces the proliferative responses of unfractionated PBMCs and highly enriched CD4 T lymphocytes when they are polyclonally stimulated through the TCR using WT31 (anti-αβ Ti chains) and anti-Leu 4 (anti-CD3) in the presence of autologous accessory cells. The addition of divalent anti-gp120/SF2 to lymphocytes previously incubated with gp120 further reduces the proliferation to the levels seen after pretreating cells with divalent anti-CD4 (anti-Leu 3a). CD8 T lymphocytes, on the other hand, show no change in TCR-mediated proliferation following preincubation with either anti-CD4 or gp120/anti-gp120. We find no evidence for significant cell death by apoptosis using methods of DNA analysis or flow cytometry and DNA-specific dyes to account for the loss of CD4 lymphocyte proliferation. Interleukin-2 restores the proliferation suppressed by gp120/anti-gp120 suggesting the induction of reversible functional anergy.