T cells with natural killer cell phenotype and function (NKT cells) have been described in both human and murine tissues. In this study, culture conditions were developed that resulted in the expansion of CD8+ NKT cells from bone marrow, thymus, and spleen by the timed addition of interferon-γ (IFN-γ), interleukin 2 (IL-2), and anti-CD3 monoclonal antibody. After 14 to 21 days in culture, dramatic expansion of CD3+, CD8+, αPT-cell receptor+ T cells resulted with approximately 20% to 50% of the cells also expressing the NK markers NK1.1 and DX5. The CD8+ NKT cells demonstrated lytic activity against several tumor target cells with more than 90% lysis by day 14 to day 21 of culture. Cytotoxicity was observed against both syngeneic and allogeneic tumor cell targets with the greatest lytic activity by the cells expressing either NK1.1 or DX5. The expanded CD8+ NKT cells produce TH1-type cytokines with high levels of IFN-γ and tumor necrosis factor α. Expansion of the CD8+ NKT cells was independent of CD1d. Ly49 molecules were expressed on only a minority of cells. A single injection of expanded CD8+ NKT cells was capable of protecting syngeneic animals from an otherwise lethal dose of Bcl1 leukemia cells. Expanded CD8+ NKT cells produced far less graft-versus-host disease (GVHD) than splenocytes across major histocompatibility barriers, even when 10 times the number of CD8+ NKT cells as compared to splenocytes were injected. This reduction in GVHD was related to IFN-γ production since cells expanded from IFN-γ knock-out animals caused acute lethal GVHD, whereas cells expanded from animals defective in fas ligand, fas, IL-2, and perforin did not. These data indicate that CD8+ NKT cells expanded in this fashion could be useful for preserving graft-versus-leukemia activity without causing GVHD.