Minor histocompatibility antigens (mHags) are immunogenic peptides from polymorphic cellular proteins that induce strong T-cell responses after human leukocyte antigen (HLA)-matched, mHag-mismatched stem-cell transplantation^,. mHags with broad or limited tissue expression are target antigens for graft-versus-host (GvH) and graft-versus-leukemia (GvL) reactivities. Separation of these activities is crucial for adoptive immunotherapy of leukemia without GvH disease. Therefore, using a skin-explant assay we investigated the in situ activities of cytotoxic T lymphocytes (CTLs) specific for the ubiquitously expressed mHag H-Y and for the hematopoietic-restricted mHags HA-1 and HA-2. H-Y-specific CTLs, visualized by tetrameric HLA–mHag peptide complexes, infiltrated male skin sections within 24 hours, induced severe GvH reactions of grade III–IV and produced high levels of IFN-γ. In contrast, CTLs specific for the hematopoietic system–specific mHags HA-1 and HA-2 induced no or low GvH reactions above background and produced little or no interferon-γ, unless the skin sections were preincubated with HA-1/HA-2 synthetic peptides. These results provide the first in situ dissection of GvH effects by mHag-specific CTLs and show that ubiquitously expressed mHags are the prime targets of GvH disease.