Freshly isolated, human peripheral blood T (PBT) cells are largely resistant to the apoptotic effects of anti-CD3 monoclonal antibody, ionomycin, or phorbol 12-myristate 13-acetate (PMA). We demonstrate here, however, that PBT cells, including both CD4+ and CD8+ cell populations, can be readily induced to undergo apoptosis when cocultured with either autologous or allogeneic monocytes (Mo) in PMA-containing medium. Incubation of PBT cells with Mo at a ratio of 1:1 for 18 hr resulted in maximal levels (80%) of apoptotic cell death. The mechanism whereby Mo enable PBT cells to undergo apoptosis in PMA-containing medium appeared to depend on cell-cell contact or close proximity between Mo and PBT cells rather than solely via soluble mediators. It was demonstrated that Mo acquire the ability to prime PBT cells for apoptosis after treatment with PMA and that treated Mo maintain this ability even after fixation with formaldehyde. It was also found that once PBT cells became primed for apoptosis by incubation with PMA-pretreated Mo, the primed PBT cells were susceptible to apoptosis triggered not only by PMA but also by either ionomycin or by monoclonal antibody crosslinking of T-cell surface molecules such as CD4 and CD3. Interestingly, the degree of apoptosis of CD4+ T cells by crosslinking of CD4 molecules via a combination of gp120, anti-gp120, and goat anti-mouse IgG was significantly greater for T cells primed with PMA-treated Mo than for unprimed T cells. Together, these findings reveal an important role for accessory cells in priming resting PBT cells for apoptosis and suggest a possible Mo-dependent mechanism by which T cells may become primed for apoptosis in human immunodeficiency virus-infected asymptomatic individuals.