A dot immunoblot screening assay was used to identify rat monoclonal antibodies to a human myeloid cell differentiation-specific nuclear antigen (MNDA). The selection was based on the positive reaction of hybridoma cell supernatants with a concentrated nuclear protein extract prepared from late stage human myeloid leukemia cells that express MNDA (HL-60) coincident with a negative reaction with the same extract prepared from a non-expressing more immature human myeloid leukemia cell line. The approach provided an efficient method for obtaining monoclonal antibodies to a specific low abundance nuclear antigen that has not been purified. Sixteen wells from three fusions contained antibody displaying a specific reaction with the nuclear protein fraction obtained from the HL-60 cells. Immunoblotting analysis revealed that all of the sixteen specific hybridoma cell lines produced antibody against the same Mr 55,000 nuclear antigen. Selecting hybridoma cells that produce antibody reactive with the native antigen provided antibody suitable for detecting MNDA in immunocytochemical tests. The rat monoclonal antibodies were purified and coupled to CNBr-activated agarose and carbonyldiimidazole-activated agarose. Although both antibody affinity matrices exhibited the same antigen binding capacities, the matrix prepared using carbonyldiimidazole-activated agarose bound the MNDA with a high level of specificity while the matrix prepared from CNBr-activated agarose bound numerous other nuclear proteins.