LONG-TERM reconstitution of the lymphohaematopoietic cells of a mouse after lethal irradiation requires the transplantation of at least (5-10) × 103 bone marrow cells1,2. Several cell-separation techniques based on cell-surface characteristics have been used in attempts to identify the pluripotent haematopoietic stem cells (PHSC), and have allowed the long-term engraftment of lethally irradiated mice with an enriched fraction of fewer than 200 marrow cells3-5. But these techniques enrich not only for PHSC but also for haematopoietic progenitors, especially day-12 spleen colony-forming units (CFU-S)3-5. Although day-12 CFU-S have been postulated to be primitive multipotential haematopoietic progenitors, with day-8 CFU-S representing later, more committed progenitors6, recent evidence suggests that neither of these CFU-S represents mouse PHSC7-9. Here we report that counterflow cen-trifugal elutriation, which sorts cells on the basis of size and density, can separate PHSC from these less primitive progenitors. The fraction containing the largest cells was enriched for the granulocyte-macrophage colony-forming units (CFU-GM), but gave only transient, early engraftment and was therefore depleted of PHSC. The intermediate fraction was enriched for CFU-S, but depleted of CFU-GM. Despite being devoid of CFU-GM and CFU-S, the fraction consisting of only morphological lymphocytes gave sustained, albeit delayed, reconstitution of all lympho-haematopoietic cells, and was therefore enriched for PHSC. We conclude that there are two vital classes of engrafting cells: committed progenitors, which provide initial, unsustained engraftment, and PHSC, which produce delayed, but durable, engraft-ment. Therefore for late haematological reconstitution, PHSC must be transplanted with a distinguishable source of early engraft-ing cells, thereby allowing the lethally irradiated host to survive initial aplasia.