In this study, we have investigated the potential mechanisms responsible for the loss of human immunodeficiency virus type 1 (HIV-1)-specific cytolytic activity in the advanced stages of HIV-1 infection. We have demonstrated that HIV-1-specific cytotoxic T lymphocytes are predominantly contained within the CD8+DR+ subset. Furthermore, we have shown by a redirected killing assay that there is a dichotomy between HIV-1-specific cytolytic activity and broad cytolytic potential since the cytolytic machinery of CD8+DR+ cells is still functioning even in patients with AIDS who have lost their HIV-1-specific cytolytic activity. In addition, by comparative analysis of these two types of cytolytic activity over time we have demonstrated a progressive loss of HIV-1-specific cytolytic activity in the advanced stages of the disease, whereas the cytolytic potential remained unchanged regardless of the clinical stage. As previously shown in patients with AIDS, even in asymptomatic HIV-1-seropositive patients, CD8+DR+ cells from the same patient, compared to CD8+DR- lymphocytes, showed a substantial reduction in their ability to proliferate in vitro in response to different stimuli, such as mitogens (phytohemagglutinin and phorbol 12-myristate 13-acetate) and monoclonal antibodies directed against CD3, CD2, and CD28 molecules, and displayed a defective clonogenic potential. Thus, on the basis of these results we propose that the loss of HIV-1-specific cytolytic activity in HIV-1-infected individuals may result at least in part from a progressive decrease in the pool of HIV-1-specific cytotoxic T lymphocytes belonging to the CD8+DR+ subset whose ability to expand has been impaired.