Fifteen bone marrow samples were obtained from consenting donors for allogenic bone marrow transplantation and were studied by flow cytometry. The simultaneous analysis, cell by cell, of size (forward angle light scattering), structure (right angle light scattering) and surface antigenic expression with 20 monoclonal antibodies allowed selection of ten reproducible cell subsets which were sorted and studied by microscopic examination in order to confirm their distinct characteristics. This very accurate analysis of a bone marrow cytogram should find applications in view of a better characterization of normal and pathological human bone marrow.