We describe the relation between clamping time and blood volume collected, and two enrichment systems of CD34+ stem cells from umbilical cord blood, in order to determine the criteria for an excellent recovery with high proliferative ability and bone marrow reconstitution. After an obstetrician-based cord blood collection, the purification of stem cells was performed either with a combination of monoclonal antibodies, negative selections, using the Stem Sep method, or with a positive cells selection owing to their surface CD34 antigens, using the Mini Macs system. An excellent recovery of haematopoietic progenitors--burst forming unit erythroid; colony-forming unit granulocyte and macrophage; and colony-forming unit granulocyte, erythroid, monocyte, and macrophage--inversely related to an increase in clamping time, was achieved using the Mini Macs system (54% of colonies with a 90% purity), while the recovery of haematopoietic progenitors was 35% (with a 80% purity) using the Stem Sep method. By clamping umbilical cord blood at an early stage, we obtained a greater number of CD34+ cells, and their clonogenic activity increased with enrichment. This is particularly useful considering that the number of CD34+ stem cells contained in a unit of placental blood is sufficient for transplanting into a child but not for an adult engraftment. Thus, using these methods, we obtain a larger number of CD34+ stem cells. This increases the possibility of reducing graft versus host disease in adult patients and produces survival rates similar to the ones we observed in transplantation of bone marrow from unrelated donors.