Rabbit antiserum was raised against leukemic null-cell line, NALL-1. After absorptions with normal human red blood cells, normal human liver cells and established leukemic T-cells (TALL-1) and B-cells (BALL-1), the antiserum reacted specifically with leukemic null-cell lines (NALL-1, KOPN-1, KOPN-8 and AL-6), but not with cells of TALL-1, BALL-1 and an acute promyelocytic leukemic cell line (HL-60). This absorbed anti-NALL-1 serum also reacted with 9 out of 12 null-acute lymphoblastic leukemia cells, 4 of 10 chronic myelogenous leukemia cells in blastic crisis and 2 of 4 null-non-Hodgkin's lymphoma cells, but not with other leukemic cells. The reactivities of the antiserum against normal peripheral blood mononuclear cells and PHA-stimulated blasts were less than 5%. These results indicate that the antigen defined by the absorbed anti-NALL-1 serum is a null-cell type leukemia-lymphoma associated antigen, but not a specific one. Thus the antiserum is a useful marker for the diagosis of null-cell type leukemia and lymphoma.