Tumor-specific antigens for leukemia cells have been sought for the past decades, but none of cell surface markers met sufficient criteria as a 'phenotypic signature'. Here we suggest that JL1 antigen can be efficiently used for diagnosis and treatment. JL1 is a human thymocyte differentiation antigen strictly confined to a CD4⁺CD8⁺ double positive subpopulation of cortical thymocytes. Despite its restricted distribution in normal tissues and cells, the expression of JL1 is highly associated with hematopoietic malignancies, particularly various types of leukemia such as T-lineage acute lymphoblastic leukemia (T-ALL), non-T-ALL, and acute myelocytic leukemia (AML). The expression of JL1 antigen was observed in 75.6% of leukemic cases (117 out of 154 leukemic patients tested) with a high mean fluorescence intensity on flow cytometric analysis and confirmed by immunoblotting. Since JL1 antigen is selectively expressed on the surface of human leukemic cells, but not on mature human peripheral blood cells and normal bone marrow cells, anti-JL1 mAb can be used as a reagent of choice in the routine diagnosis of various types of leukemia, providing an excellent candidate for the treatment of these diseases.