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The aim of this study was to characterize the lymphocyte and the hematopoietic stem and progenitor cell (HPC) subsets of cryopreserved premature cord blood (PCB) compared to term cord blood (TCB) by flow cytometry, to study the influence of birth conditions, and to assess its availability for transplantation.Four-color flow cytometric analysis was performed on 43 PCB and 40 TCB cryopreserved samples using a panel of 24 different mAbs, directed against lymphoid and HPC surface markers. The CB volume was estimated by the weight of the newborn to determine the absolute MNC and CD34+ cell content/CB sample. Clinical and obstetrical data were recovered. Statistical comparisons and a multiple regression analysis were performed.No consistent differences were found in the mononuclear cell (MNC) or CD34+ cell concentration (×106/L) between PCB and TCB. The percentage of primitive HPC (CD34+CD38-, CD34+CD38-CD90-HLA-DR-, CD34+CD38-CD90-HLA-DR+) and primitive lymphoid progenitors (CD34+CD7+, CD34+CD7+CD19-CD117-) were higher in PCB than in TCB. Correspondingly, TCB had an increased percentage of committed HPC. No sample of PCB contained >2 × 107 MNC/kg (and only 48% had >1 × 105 CD34+ cells) for a recipient of 20 kg body wt, as the minimum threshold recommended for CB transplantation. Obstetrical factors modulated mainly lymphocyte subsets and fewer HPC subpopulations. Acute fetal distress increased CD34+ cells, especially the immature subsets. Maternal treatment with dexamethasone and intrauterine growth retardation decreased CD3+ cells. No other obstetrical factors played a detrimental effect on CB cells if used for transplantation.PCB is richer in immature cells both in lymphocyte and HPC populations, and its use for transplantation, at least in special cases, should be reconsidered. Am. J. Hematol. 81:397-410, 2006. © 2006 Wiley-Liss, Inc.