Monoclonal antibodies produced by hybridoma lymphocytes can have high specificity for cell membrane antigens present on distinctive cell types. Hybridoma technology has been employed in our laboratory to produce antibodies which bind selectively to hematopoietic progenitor cells. Because these cells are unavailable in quantities sufficient for immunization, the K562 leukemia cell has been employed as an antigen because its properties are similar to those of the true hematopoietic stem cell. Antibodies raised in mice against K562 cells show a variety of binding patterns to normal cells, as assayed by various immunological methods. A small number of these antibodies are inhibitory to hematopoietic progenitors. Anti-K562 cell antibodies may prove useful in disease classification, the detection of stem cells by immunologic characteristics, and the fractionation of bone marrow cells by fluorescent cell sorting. The same methods are being applied to the production of monoclonal antiboides to mouse cells in an attempt to isolate the murine stem cell by immunological means, and human-mouse lymphocyte hybridomas are being produced to isolate and characterize pathologic antibodies which occur in hematologic disease.