Human umbilical cord blood (HUCB) is a possible alternative to bone marrow (BM) and mobilized peripheral blood (PB) for transplantation of hematopoietic progenitors. The aim of this study was to evaluate the phenotypic profile of CD34+ progenitors present in HUCB. A flow cytometric analysis was performed on 20 HUCB samples, using a large panel of monoclonal antibodies recognizing different lineage or activation antigens, in double labeling with CD34. A toal of 13,897+/-2,529 cells/microL, 0.84+/-0.83% of which were CD34+, was found. The large majority of CD34+ cells were committed toward initial myeloid differentiation (CD33+, CD13+) and expressed the transferrin receptor (CD71). A substantial proportion of these cells (about 40%) co-expressed CD45RA and CD117, while a very small number displayed markers of advanced myeloid commitment, such as CD14, CD15 and CD41 (less than 2%), or those of lymphoid differentiation: CD2, CD5, CD7, CD10 and CD19 (less than 6%). About 11% of HUCB CD34+ cells were primitive progenitors, as suggested by the absence of HLA-DR and CD38 on their surface. As previously observed in BM and mobilized PB, the phenotype of HUCB CD34+ cells is quite heterogeneous. In particular, HUCB contains subpopulations of both early and committed hematopoietic progenitors which may represent a valid source for transplantation.