Human monocytes infected in vitro with human immunodeficiency virus (HIV) soon after adherence to plastic substrate demonstrated a significantly decreased ability to restimulate autologous immune T-lymphocyte proliferation after exposure to soluble (tetanus toxoid) and particulate [herpes simplex virus (HSV)] antigen. Incubation with the cyclo-oxygenase inhibitor, indomethacin (2-5 microM), prevented inhibition of antigen-stimulated lymphocyte proliferation. The inhibitory activity was identified in ultrafiltrates containing the low molecular weight fraction (less than 3000 MW) of supernatants from HIV-infected monocyte cultures. This activity was significantly and markedly reduced in similar ultrafiltrates prepared from indomethacin-treated cultures. Increased concentrations of prostaglandin E2 (PGE2) were detected in ultrafiltrates from HIV-infected monocyte cultures compared with uninfected cultures and cultures preincubated with indomethacin. Ultrafiltrates were inhibitory when added during the presentation of antigen to T lymphocytes but not when removed from monocyte cultures prior to the addition of lymphocytes. In addition, ultrafiltrates inhibited antigen-stimulated lymphocyte proliferation and PHA-induced lymphocyte proliferation to the same extent. These data indicate that cyclo-oxygenase products of arachidonic acid, including PGE2, are produced in excess by HIV-infected monocytes and that PGE2 and perhaps other cyclo-oxygenase products are implicated in the inhibition of antigen-stimulated lymphocyte proliferation via a direct effect on T lymphocytes.