Two monoclonal antibodies (NH-1 and NH-2 abs) were produced by immunizing BALB/c mice with a null-acute lymphoblastic leukemia (null-ALL) cell line, NALL-1, and fusing the mouse spleen cells to mouse myeloma cell line, P3-NS-1/1-Ag4-1. The resulting hybrid cells were screened, and two stable monoclonal ab-producing cell lines were isolated by repeated clonings. Reactivities of the abs were analyzed using an indirect membrane immunofluorescence test against cultured cell lines, normal peripheral blood cells, bone marrow cells, lymph node cells, spleen cells, hematopoietic tumor cells and non-hematopoietic tumor cells. NH-1 ab reacted exclusively with monocytes and leukemia cells of null-All, acute myelocytic leukemia, acute promyelocytic leukemia, acute monocytic leukemia and chronic myelocytic leukemia in blast crisis (CML-BC). NH-2 ab did with leukemia cells from some cases of null-ALL and CML-BC. Mature granulocytes and lung adenocarcinoma cells also reacted with NH-2 ab. When HL-60 cells were cultured with dimethylsulfoxide, expression of NH-1 reactive antigen (NH-1 ag) on the differentiated HL-60 cells was decreased, but NH-2 ag was not expressed. When NALL-1 cells were cultured with NH-1 or NH-2 ab, NH-2 ag modulated, but NH-1 ag did not. Immunoglobulin of both abs was IgG and did not exhibit complement-dependent cytotoxic activity. These results indicate that NH-1 ag was a differentiation ag expressed commonly by immature hematopoietic tumor cells and monocytes, and NH-2 ag may be a unique common ALL ag which is also expressed on mature granulocytes and lung adenocarcinoma cells.