It has been shown that T cell-mediated immunity plays a major role (4,5,(7)(8)(9)(10)12) in control of and recovery from herpes simplex virus (HSV) infection. Nevertheless, the mechanism of immune T cell-mediated protection in vivo is complex and poorly understood.Recent studies have revealed that peripheral T cells express different Lytalloantigens on their surfaces and are divisible into three subpopulations with phenotypes Lyt-1.2.3, Lyt-l, and Lyt-2.3 (2). Lyt-1.2.3 cells are presumed to be precursors for Lyt-I or Lyt-2.3 cells (2). Lyt-l cells are programmed to help or amplify activities of other cells after stimulation by an antigen (2). They help B cells to produce antibody and activate macrophages in the generation of the delayed type hypersensitivity reaction (2). Lyt-2.3 cells are known to suppress antibody production and other immune responses, and to be cytotoxic cells (2).In this study, we have used Lyt-alloantisera for the examination of immune T lymphocytes responsible for providing protection in mice lethally infected with HSV.Six-to 10-week-old athymic nude (nu/nu) mice and their littermates (nu/+) with BALB/c genetic background were purchased from the Central Laboratories of Experimental Animals Co., Ltd., Japan. When athymic nude mice were injected intracutaneously with 2.5 X 10 4 plaque-forming units (PFU) of the virulent Hayashida strain of HSV-l per 0.05 ml, all of the mice died after development of severe zosteriform skin lesions, while all of the control nul + mice survived the infection, as reported previously (10), indicating that resistance of HSV infection is T-cell dependent.In order to determine the phenotypes of committed lymphocytes responsible for HSV resistance, immune spleen cells were transferred to numu mice that were lethally infected with HSV, following treatment with various antilymphocyte sera. Anti-lymphocyte alloantisera were obtained from Cederlane Laboratories, Canada. Anti-Thy-1.2 alloantisera were produced in AKR/J mice by injecting thymocytes