Allogeneic peripheral blood stem cell transplantation (PBSCT) is primarily limited by graft-versus-host disease (GVHD). In murine models, we have demonstrated that donor CD4+ T cells of Th1 cytokine phenotype (defined by their secretion of IL-2 and IFN-gamma) mediate GVHD. In contrast, donor CD4+ T cells of Th2 phenotype (defined by their secretion of IL-4, IL-5, and IL-10) do not generate GVHD, and abrogate Th-1-mediated GVHD. Importantly, we have demonstrated that enrichment of murine allografts with Th2 cells reduces GVHD without impairing the ability of donor T cells to prevent graft rejection. These studies indicate that the administration of Th2 cells after allogeneic transplantation represents a strategy for achieving alloengraftment with reduced GVHD.