W e investigated whether minor histocompatibility (mH) antigen-specific cytotoxic T lymphocytes (CTL) can discriminate between leukemic hematopoietic progenitor cells (leukemic-HPC) from AML or CML patients, the HPC from their remission bone marrow (remission-HPC), and normal HPC from their HLA-identical sibling bone marrow donor (donor-HPC). Specific lysis by CD8+ CTL clones was observed not only of the leukemic-HPC but also of the donor-HPC in 3/4 patient/donor combinations expressing mH antigen HA-1, 3/5 combinations expressing mH antigen HA-2, 2/3 combinations expressing mH antigen HA-3, and 2/2 combinations expressing mH antigen HY-A1. In four patient/donor combinations the recognition of the donor-HPC was clearly less than of the leukemic-HPC, indicating differential susceptibility to lysis by these mH CTL clones. In addition, differential recognition of leukemic-HPC and remission-HPC within seven patients was analyzed. In one patient expressing the HA-2 antigen on the leukemic cells the recognition of the remission-HPC was clearly less than of the leukemic-HPC. One CD4+ CTL clone showed specific lysis of the leukemic-HPC from an AML patient and a CML patient as well as of normal remission-HPC and donor-HPC. These results illustrate that in general CD8+ and CD4+ mH antigen specific CTL clones do not differentially recognize leukemic-HPC and normal-HPC. However, differences in susceptibility to lysis of malignant versus normal cells may contribute to a differential GVL effect.