Even though there has been considerable progress in the phenotypic characterization of CD34+ bone marrow cells, there is still limited knowledge about the cell phenotypes corresponding to functional terms such as colony-forming cells, burst-forming cells, long-term culture-initiating cells, and high-proliferative potential cells. In this study, we performed a detailed analysis of phenotypic characteristics of subsets of CD34+ cells. We compared cells from adult and fetal bone marrow to investigate whether reported functional differences are reflected in the cellular phenotypes. CD34+, CD38-/lo, HLA-DR+ cells, which have been shown to contain the most immature hematopoietic progenitor cells, stained as a homogeneous population with most monoclonal antibodies (mAbs). The antigens sLex, CD32, and CD7 were, however, heterogeneously expressed in the CD38-/lo population. Phenotypic differences in the CD34+, CD38-/lo population was found when comparing adult and fetal bone marrow cells. Adult bone marrow CD34+, CD38-/lo cells stained more brightly with CD4, Thy-1, and CD49b and more dimly with CD32 than the same population in fetal bone marrow. Certain antigens that have previously been regarded as lineage-specific were found on the CD34+, CD38-/lo, HLA-DR+ cells in both fetal and adult bone marrow. These included CD52, CD13, and CD33. The markers that were found to be most useful in discriminating between subsets of lineage-committed cells within the CD34+, CD38+ population included the B cell marker CD19 and the granulomonocytic marker CD64. The phenotypic analysis presented here should provide a basis for establishing a better link between functional and phenotypic characteristics of hematopoietic progenitor cells.