We describe a patient with acute myeloid leukemia (AML) who developed polyclonal large granular lymphocyte (LGL) proliferation. The reciprocal evolution of AML and LGLs suggested that these LGLs had an anti-tumor activity. The patient's LGLs killed autologous leukemia cells in a different way to classical T lymphocyte-mediated cytotoxicity since it did not rely on the recognition of target antigens presented by major histocompatibility complex (MHC) class I molecules by the CD3/TcRalphabeta complex. This killing was also different from natural killer (NK)-mediated cytotoxicity, which depends on the absence of MHC class I molecule recognition by NK inhibitory receptors. The LGLs were polyclonal, had a CD3+/CD8+/CD56+ phenotype, and did not express the natural killer cell receptors (NKRs) for MHC class I molecules. The LGLs did not express the NK-specific activating natural cytotoxicity receptors but expressed the 2B4 non-MHC restricted triggering receptor, whose ligand CD48 was expressed by leukemic cells and normal bone marrow cells. The 2B4 receptor participated in the ability of LGLs to lyse patient's leukemia. This represents a novel function for 2B4 in man, since this molecule, at variance with the murine system, was considered not to have direct effects on CD8+ T cell-mediated cytotoxicity. This case report allowed us to describe a novel T lymphocyte-mediated anti-tumor mechanism which relied on (1) the abnormal expansion of the rare 2B4-positive CD3+/CD8+/CD56+ T lymphocyte subset, (2) an as yet undescribed cytotoxicity mechanism in man which depended on 2B4 molecule. The relevance of this observation in human cancer immunotherapy has to be further investigated.