Perforin-deficient (-/-) mice were used as T-cell donors for infusion into irradiated major histocompatibility complex (MHC)-disparate recipients to investigate the requirement for perforin-mediated cytolysis during graft-versus-host disease (GVHD) generation. Administration of 5× 10 6 C57BL/6 (H2 b) perforin-/-splenocytes was significantly less effective in inducing GVHD lethality when given to MHC class I+ II disparate B10. BR (H2 k) recipients, as compared with wild-type (+/+) controls. Perforin expression by donor T cells was not required for GVHD induction because recipients given fivefold higher numbers of perforin-/-donor splenocytes uniformly succumbed to lethal GVHD. Because both CD4+ and CD8+ donor T cells are required for optimal GVHD lethality in this strain combination, to discern the relative contribution of perforin-mediated cytolysis by CD4+ and CD8+ T cells, additional studies were performed. For these latter studies, we used a sensitive assay involving the infusion of highly purified CD4+ or CD8+ T cells into sublethally irradiated MHC class II or I disparate recipients, respectively. As compared with recipients of perforin+/+ T cells, recipients of either CD4+ or CD8+ perforin-/-T-cell subsets had a significant reduction in GVHD-mediated lethality at T-cell doses that were uniformly lethal. T-cell dose titration studies established that GVHD lethality in recipients of perforin-/-CD4+ or CD8+ T cells was reduced by approximately threefold. These data are the first to indicate that approaches to limit perforin-mediated cytolysis should be similarly effective in situations in which CD4+ or CD8+ T cells dominate the GVHD response.