Bone marrow transplantation is limited by the lack of HLA-matched donors and the frequent occurrence of GvHD. Hematopoietic transplants using CB cells are being increasingly used in pediatric patients. Early estimates suggest that there may not be enough hematopoietic progenitor cells in an average cord blood sample to reconstitute adult patients. We investigated the in vitro proliferative potential of CB CD34+ enriched fraction. We studied the immunophenotypic characteristics of UCB mononuclear cells (n=30) by one and two color flow cytometry. CD34+ cells were isolated using a Ceprate LC-34 Biotin kit. In clonogenic assays CFU-GEMM, CFU-GM and BFU-E were quantified by methylcellulose culture in both mononuclear cells and CD34+ enriched fractions after the addition of different combinations of Epo, IL3, GM-CSF, G-CSF and SCF. The CB cells were characterized by a low proportion of CD3+ T cells, increased CD4/CD8 ratio, minimal expression of HLA-DR and increased proportion of CD5/CD19 double possitive B cells. All these do reflect the immaturity of CB lymphocytes contributing to the observed rate of GvHD following transplantation. The number of CFU-GM, CFU-GEMM, BFU-E in cultures of CD34+ enriched cells had increased 75-150-400 fold respectively over the cell cultures of mononuclear cells. The addition of SCF increased the number of CFU-GM and CFU-GEMM colonies 3 fold in the CD34+ enriched fraction whereas it had no influence upon the mononuclear cells fraction. The influence of SCF was minimal on the BFU-E formation in both CD34+ enriched and mononuclear cell fractions when added to GM-CSF, IL3 and Epo. We noticed though that the combination of Epo and SCF increased by 3,7 the BFU-E numbers in the CD34+ enriched fraction. These data demostrate that CD34+ enriched UBC progenitor cells have greater clonogenic capacity compared to UBC mononuclear cells and they are more sensitive to the addition of SCF. We therefore suggest that cord blood can be a valuable alternative source of hemopoietic stem cells and further investigation is necessary for establishing the use of ex vivo expanded cord blood progenitors as efficient graft for adult patients.