Possible effects of the ABO blood group on the proliferative and self-renewal capacity of umbilical cord CD34+ cells were evaluated. A, B and O (all Rh D+) CD34+ cells isolated from three placental blood samples were cultured in four platforms with hematopoietic growth factors on a 3-dimensional biocompatible matrix. Results from this study suggest that proliferation of CD34+ cells with the O phenotype may be greater than that of cells with the A or B phenotypes. Further ex vivo studies are required to confirm this finding and to determine the effect of the number and type of other genetically determined cell surface antigens on the capacity of hematopoietic stem cells to respond to cytokines. In addition, clinical studies aimed at determining if the CD34+ donor blood group affects the time to functional hematological reconstitution are recommended.