Peripheral blood mononuclear cells (PBMNC) from healthy donors immune to the soluble antigens tetanus toxoid (TT) and/or keyhole limpet hemocyanin (KLH) were exposed to infectious human T lymphotropic virus, type III/lymphadenopathy-associated virus (HTLV-III/LAV) with and without prior activation by TT or KLH. After exposure to the virus, PBMNC that had been activated by antigen were 10 to 100 times more susceptible to viral replication, as estimated by measurement of production of reverse transcriptase and viral antigens, than PBMNC that had been preincubated without antigen. In addition, exposure of PBMNC to HTLV-III/LAV led to a loss of lymphocyte blastogenic responses after 2 to 3 wk in culture. HTLV-III/LAV-induced inhibition of lymphocyte blastogenic responses occurred in the absence of detectable production of RT but required the use of live rather than heat-inactivated virus. These results demonstrate that HTLV-III/LAV infection is amplified by antigen-induced activation of PBMNC, and that low levels of HTLV-III/LAV infection in vitro can suppress lymphocyte blastogenic responses. This study provides an in vitro model for the analysis of HTLV-III/LAV-induced immune defects.