Varicella-zoster virus (VZV)-specific cytotoxic T cell responses were studied in 35 presumably immune and two nonimmune adult donors and in two patients with herpes zoster. Peripheral blood lymphocytes (PBLs) were stimulated for five or 12 days with autologous, irradiated, VZV-infected PBLs. Cytotoxicity was measured in a chromium-release assay. Target cells were usually cryopreserved, phytohemagglutinin-stimulated PBLs, either infected with VZV or uninfected. In testing the presumably immune adults, VZV-specific cytotoxicity was observed in 32 (91%) of 35 cases after five days and in 19 (90%) of 21 cases after 12 d of stimulation. Lysis of HLA-matched target cells was significantly greater than that of mismatched, VZV-infected target cells after both intervals. Responses were similar when PBLs from two patients with acute zoster were tested after in vitro stimulation and in one of those two tested without in vitro stimulation.