Background and Objectives. Hematopoietic progenitor cells that express CD34 are heterogeneous in their lineage affiliation and degree of maturation. Expression of CD13 and CD33 antigens indicates myeloid lineage association, but the precise sequence of expression of these two markers during differentiation is unclear. We noted the presence of CD34(+) cells expressing CD13 but lacking CD33, a subset of cells not yet well characterized. In this report we describe the prevalence and the immunophenotype of this cell subset. Design and Methods. We studied the immunophenotype of immature myeloid cells in human bone marrow samples from 11 healthy transplantation donors and in 4 cord blood samples. We used four-color flow cytometry and a large panel of monoclonal antibodies directed against lineage and differentiation-associated antigens. Three additional bone marrow samples were analyzed after immunomagnetic sorting of 034 cells. We focused our analysis on the subset of cells defined by the expression of CD34 and CD13 and the lack of CD33. Results. We found CD34(+), CD13(+), CD33(-) cells in all 11 bone marrow and 4 cord blood samples studied. These cells represented 0.5+/-0.5% (mean +/- SD) and 0.8+/-1.2% of mononucleated cells, respectively. CD34(+), CD13(+), CD33(-) cells appeared to be more immature than those expressing CD33 because of their light scatter characteristics (smaller size and lower granularity), the expression of markers associated with early hematopoietic cells (CD90, CD133 and CD117), and the absence of lineage-associated markers. Interpretation and Conclusions. These findings suggest that the expression of CD13 precedes that of CD33 during myeloid differentiation, and that CD34(+), CD13(+), CD33(-) cells are at an early stage of human myeloid cell differentiation. (C)2002, Ferrata Storti Foundation.