Following bone marrow transplantation, delayed donor leukocyte infusions (DLIs) can induce graft-versus-leukemia (GVL) effects without graft-versus-host disease (GVHD). These antitumor responses are maximized by the presence of host hematopoietic antigen-presenting cells (APCs) at the time of DLI. Using a tumor-protection model, we demonstrate here that GVL activity following administration of DLIs to established mixed chimeras is dependent primarily on reactivity to allogeneic MHC antigens rather than minor histocompatibility or tumor-associated antigens. CD8⁺ T-cell–dependent GVL responses against an MHC class II–negative tumor following delayed DLI require CD4⁺ T-cell help and are reduced significantly when host APCs lack MHC class II expression. CD4⁺ T cells primed by host APCs were required for maximal expansion of graft-versus-host reactive CD8⁺ T cells but not their synthesis of IFN-γ. In contrast, the GVL requirement for CD4⁺ T-cell help was bypassed almost completely when DLI was administered to freshly irradiated recipients, indicating that the host environment is a major factor influencing the cellular mechanisms of GVL.