Lymphokine activated killer (LAK) cells have been utilized as a useful tool in cancer adoptive immunotherapy. The lineage origin of this population has always been controversial since it shares phenotypic markers with both myelomonocytic cells and T lymphocytes. Recently we described a new monoclonal antibody (MoAb), termed LAK1, which recognizes a 120 Kd surface molecule expressed on human large granular lymphocytes (LGL) and LAK precursors and effectors. LAK1 MoAb defines two different populations of positive cells amongst peripheral lymphocytes: the first subset (20%), represented by brightly stained cells, belongs to the non T-LGL population, whereas the second subset (30%) displays low fluorescence intensity and was partially composed of T lymphocytes. More interestingly, LAK1 is shared by some other cell types, such as monocytes and vascular endothelial cells. Immunohistochemical staining performed on muscle, endometrium and lymphoid or other non lymphoid tissues shows that LAK1 antigen is selectively expressed by the reticuloendothelial system.