Abstract: Patients who develop increased numbers of ??+ cytotoxic T lymphocytes 2–6 months after allogeneic bone marrow transplantation are less likely to relapse than those who do not. The ??+ T cells isolated from blood of patients with increased ??+ T cells are CD3+CD4?CD8?CD57+, cytolytic to K562 cells, and express the V?1 T cell receptor phenotype. Similar ??+ T cells can be generated in vitro by culture of donor mononuclear cells which are enriched for ??+ T cells by immunomagnetic depletion of depleted of CD4+ and CD8+ cells. This ??-enriched cell preparation was cultured on a combination of immobilized pan-? monoclonal antibody and irradiated recipient B cell leukemia. After four weeks, the cultures were almost exclusively V?1+CD3+CD4?CD8? cells that co-expressed activation-associated antigens CD69, CD25, and HLA-DR.