Whether T-cell antigen receptors (TCR) on donor T cells require direct interactions with major histocompatibility complex class I or class II (MHCI/MHCII) molecules on target cells to mediate graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) is a fundamental question in allogeneic stem-cell transplantation (alloSCT). In MHC-mismatched mouse models, these contacts were not required for GVHD. However, this conclusion may not apply to MHC-matched, multiple minor histocompatibility antigen-mismatched alloSCT, the most common type performed clinically. To address this, we used wild-type (wt)-->MHCI-/- or wt-->MHCII-/- bone marrow chimeras as recipients in GVHD experiments. For GVL experiments, we used MHCI-/- or MHCII-/- chronic-phase CML cells created by expressing the BCR-ABL cDNA in bone marrow from MHCI-/- or MHCII-/- mice. TCR/MHCI contact was obligatory for both CD8-mediated GVHD and GVL. In contrast, CD4 cells induced GVHD in wt-->MHCII-/- chimeras, whereas MHCII-/- mCP-CML was GVL-resistant. Donor CD4 cells infiltrated affected skin and bowel in wt-->MHCII-/- recipients, indicating that they mediated GVHD by acting locally. Thus, CD4 cells use distinct effector mechanisms in GVHD and GVL: direct cytolytic action is required for GVL but not for GVHD. If these noncytolytic pathways can be inhibited, then GVHD might be ameliorated while preserving GVL.