We sought to identify imbalances of immune regulatory cells that might contribute to the depression of cell-mediated immunity that occurs during an episode of herpes zoster. Peripheral blood mononuclear cells (PBMC) were obtained from patients with herpes zoster during the acute (less than 7 days after disease onset) and convalescent (more than 10 days after disease onset) phases of illness and from healthy seropositive donors. The PBMC were analyzed for: 1) lymphoproliferative responses to varicella-zoster virus (VZV) antigens, 2) Leu-3 (helper/inducer):Leu-2 (cytotoxic/suppressor) ratios, and 3) percentages of suppressor cells as defined by co-expression of the Leu-2 and OKM1 antigens. Significantly depressed proliferative responses of VZV antigens and Leu-3:Leu-2 ratios, and increased percentages of Leu-2+ OKM1+ suppressor cells were observed in PBMC of acute phase herpes zoster patients as compared with the PBMC of convalescent patients or healthy donors. These differences were also observed in individual patients sequentially studied during both phases of disease. Cryopreserved acute phase PBMC suppressed the proliferative response of autologous convalescent phase PBMC to VZV antigens, but not to herpes simplex virus (HSV) antigens. The acute phase PBMC suppressor cell was radiation sensitive and was identified as a Leu-2+ cell by fluorescence-activated cell sorting. Thus, depression of cell-mediated immunity during the acute phase of herpes zoster was associated with a relative increase of lymphocytes expressing a suppressor cell phenotype and the activation of a radiosensitive Leu-2+ suppressor cell with some degree of antigen specificity.