Relative levels of cytoplasmic aldehyde dehydrogenase (ALDH) were determined in selected subpopulations of normal human bone marrow cells using a flow cytometric assay that simultaneously detects a cell surface antigen (as a marker of cell lineage and developmental stage) and the level of ALDH. The intracellular level of this enzyme has been shown to be directly related to cellular resistance to activated cyclophosphamide and is believed to be important in the survival of cells capable of repopulating marrow in autologous bone marrow transplant procedures. Western blot analysis and flow cytometric analysis of four murine cell lines with known ALDH levels were used to establish the relation between ALDH content and fluorescence with an affinity-purified anti-mouse ALDH antibody. An affinity purified anti-human ALDH antibody, characterized by immunoblotting of cytosolic extracts of cell lines with known ALDH content, was used to determine relative ALDH levels in the marrow subpopulations. We found that hematopoietic progenitor cells express the highest level of ALDH, while lymphocytes express the lowest level. Immature erythroid cells express ALDH at a level intermediate between progenitor cells and lymphocytes.