Three monoclonal antibodies, designated H-1, H-2 and H-3, were produced by immunizing BALB/C mice with cells of an established human null-acute lymphoblastic leukemia (NALL-1) cell line and fusing spleen cells from these mice with cells of a mouse myeloma cell line, P3-NSl/1-AG4-1. The resulting hybrid cells were initially screened for production of antibodies by complement-dependent cytotoxicity. Stable monoclonal antibody-producing cell lines were isolated by repeated cloning. These independently derived monoclonal antibodies showed complement-dependent cytotoxicities. Further characterization of the monoclonal antibodies was made by immunofluorescence. H-1 antibody reacted with all tested cells of the human nucleated cells except for HLA-lacking Daudi and K562 cells, and thus was shown to be specific for a common antigenic dererminant of HLA-A,-B and-C. H-2 antibody reacted with all of the tested null-cell lines, B-cell lines, B-cells and monocytes. Moreover, studies on fresh leukemia and lymphoma cells revealed that tumor cells from null-acute lymphoblastic leukemia, B-chronic lymphocytic leukemia and null-non-Hodgkin's lymphoma were H-2 antibody reactive. Cells from some patients with acute myelogenous leukemia, acute monocytic leukemia, chronic myelogenous leukemia in blast crisis and abult T-cell leukemia were also H-2-positive. In contrast, T-acute lymphoblastic leukemia, T-nonHodgkin's lymphoma cells and mature T-cells did not react with H-2. A cross absorption study using H-2 and anti-HLA-DR monoclonal antibody confirmed the same specificity. Interestingly, H-2 showed blocking activities against the stimulation in one-way mixed lymphocyte culture. These results indicate that H-2 is a monoclonal antibody specific for a framework determinant of human Ia-like antigens. Reactivity of H-3 antibody was different from that of H-1 and H-2. H-3 reacted with some T-cells, B-cells and monocytes, suggesting that the antibody is associated with human leukocyte common antigens.