This study describes the inhibitory effect exerted by activated CD8+ T cells on the replication of HIV in naturally infected CD4+ T cells. Highly purified CD4+ T cells from asymptomatic HIV seropositive individuals were stimulated with anti-TCR mAb-coated beads in the presence of IL-2. HIV was subsequently reproducibly isolated in cell supernatants from all study participants (53 cultures from 42 individuals). Both autologous and allogeneic CD8+ T cells from asymptomatic HIV seropositive and healthy HIV seronegative individuals inhibited the replication of HIV in these cultures in a dose-dependent manner. CD8+ T cells from patients with AIDS showed reduced or no such inhibitory activity. The inhibitory effect was not dependent on direct cell-cell contact: an inhibitory effect was exerted by CD8+ T cells across a semipermeable membrane, and an inhibitory activity was also exerted by the cell-free supernatants from activated CD8+ T cells. These results suggest that activated CD8+ T cells secrete a soluble inhibitor of HIV replication.