Cryopreserved human umbilical-cord (HUC) blood is an alternative to bone marrow as a source of haemopoietic "stem" cells for HLA-identical transplantation of children with leukaemia or Fanconi's anaemia. We have studied the in-vitro growth potential of HUC blood in clonogenic assays and in longterm haemopoietic cultures. Clonogenic assays showed that HUC blood produced as many haemopoietic-cell colonies as normal adult bone marrow and a higher proportion of primitive-cell colonies. In longterm culture on preformed irradiated marrow stroma, both progenitor-cell production and lifespan of cultures were significantly greater in HUC blood than in normal bone marrow (p = 0.0007). Our findings indicate that the quality and quantity of HUC-blood-derived haemopoietic "stem" cells are better than those of normal bone marrow. Therefore, single HUC-blood donations are probably sufficient for adults requiring transplantation for leukaemia and other haemopoietic disorders. Banking of HLA-typed HUC blood to facilitate transplantation of patients who lack a family donor should be considered.