BACKGROUND Hematopoietic progenitor cells (HPC), identified by expression of the CD34 surface antigen, show morphological and phenotypic heterogeneity in bone marrow (BM) and peripheral blood (PB). METHODS CD34+ HPC subpopulations present in 18 PB leukaphereses after high-dose chemotherapy in cancer patients and in 11 BM samples from patients with stage IA lymphoma were characterized. In order to identify CD34+ HPC subsets within these two compartments, the expression of lineage- or activation-associated antigens and the c-kit gene product (CD117) was studied by flow cytometry, using a large panel of monoclonal antibodies (MoAb) in double labelling. RESULTS We observed a higher proportion of CD34+/CD13+ and CD34+/CD33+ cells (myeloid commitment) in harvested leukapheresis products than in BM. On the contrary, a higher percentage of CD34+/CD10+ and CD34+/CD19+ cells (B-lymphoid commitment) was found in BM. The percentage of the most immature subset of CD34+ HPC (CD38- and HLA-DR-) was also higher in BM than in mobilized PB. No differences in proportions were found with respect to the expression of CD14, CD15, CD45RA (myeloid commitment), CD2, CD5, CD7 (T-lymphoid commitment), CD117, CD71 and CD45RO antigens. In terms of absolute values, however, significantly higher amounts of CD34+ HPC co-expressing CD13, CD33, CD5, CD7, CD71, CD117, CD45RA, CD45RO were detected in leukaphereses than in BM. The absolute number of immature HPC (CD34+/CD38- and CD34+/HLA-Dr-) was also significantly increased in mobilized PB. CONCLUSIONS Our data confirm the heterogeneous phenotypic profile of HPC, thus supporting the hypothesis that different CD34+ subpopulations may have clinical relevance on the rapidity and long-term durability of engraftment in patients who undergo high-dose chemotherapy followed by rescue with HPC. We also demonstrated that mobilized PB is a particularly rich source of both primitive and committed HPC, more than BM.