Patients with acquired immunodeficiency syndrome (AIDS) commonly suffer from opportunistic infections associated with members of the herpes virus family. To investigate whether certain of these other viruses might have an effect on the ability of the human immunodeficiency virus type 1 (HIV-1) to replicate, we coincubated peripheral blood mononuclear cells (PBMC) from nine HIV-1-seropositive donors with live preparations of various herpes viruses. In seven of nine cases, exposure of PBMC to preparations of either HSV-1, HSV-2, or CMV stimulated the cells to become active producers of HIV-1, as determined by reverse transcriptase activity and by the presence of infectious progeny virus. This increased production of HIV-1 particles appeared to be a consequence of mitogenic proliferation and of herpes virus-encoded transacting factors. These results supplement earlier findings on the molecular activation of the HIV-1 genome by both HSV and CMV genetic elements and point to a possible role for these viruses in the pathogenesis and ultimate clinical outcome of HIV-1 infections.