This study was undertaken with the aim of elucidating the mechanisms underlying the cell-mediated immunodeficiency seen in the acquired immunodeficiency syndrome (AIDS). An intrinsic functional defect in the in vitro surviving T lymphocytes from patients with AIDS has been described. This defect is reflected by profound reductions in both the cloning efficiency of these cells and in the number of precursor cells for response to lectins. Since many patients affected by AIDS present active cytomegalovirus (CMV) infections and impairment in CMV-specific cellular immunity, we examined the number of CMV-specific precursor cells in patients affected by the AIDS-related complex (ARC), who had serum antibodies to CMV and to the human-T-lymphotropic retrovirus-type III (HTLV-III), recently termed human immunodeficiency virus (HIV). Their responses were compared to those of patients with AIDS and to those of healthy-CMV-seropositive and HTLV-III seronegative controls. We detected a significant reduction of precursors for cell-mediated immune response to CMV in AIDS, in comparison to normal controls and a reduction in ARC, even if not significant. In parallel, we assayed the response to phytohemagglutinin, which was maintained in ARC and depressed in AIDS. Our results show a defect of specific cell-mediated immunity to CMV in ARC and AIDS patients.