Expression of the protooncogene, c-myb, in various subpopulations of normal human hematopoietic cells was characterized. Cells expressing the immature cell surface marker, CD34 (My10), were isolated by immune adherence with the “panning” technique or immunomagnetic microspheres and were shown to be strongly positive for c-myb protein expression in an immunoperoxidase assay. The CD34+ progenitor cell population was further separated into myeloid plus erythroid progenitors (CD34+, CD10- ) v B-lymphoid precursors (coexpressing CD34 and CD10) by two-color FACS. Both CD34+ progenitor cell subsets strongly expressed c-myb protein by the immunoperoxidase assay. A flow cytometric assay was then developed which permitted simultaneous detection of a cell-surface antigen (to characterize lineage and stage of maturation) and the nuclear oncoprotein. This assay confirmed that CD34+ cells were strongly positive for c-myb expression and also allowed quantitative comparisons of c-myb expression in selected populations of other normal hematopoietic cells. Most human bone marrow cells appear to express some level of c-myb protein, although the CD34+ progenitor cell population expresses the highest amount.