The recovery of circulating haemopoietic progenitor cells was evaluated serially in seven patients for 3-4 weeks after bone marrow transplantation (two autologous and five allogeneic) as treatment for leukaemia. Eight normal healthy volunteers were used as controls. CFU-G (colony forming unit-granulocyte) was found to be the earliest progenitor cell to recover at a mean interval of 16 +/- 1 (SE) days post-transplantation. A lag of 7 days was found before circulating CFU-GM (colony forming unit-granulocyte, monocyte) reappeared, while BFU-E (burst forming unit-erythroid) were detectable in only two patients in the first 4 weeks. The peak level of circulating progenitors was very low, 28 +/- 8/ml, compared with a mean level of 619 +/- 235/ml in eight normal individuals. This pattern of circulating progenitor cell recovery post-transplantation was consistently seen in all patients. CFU-G reappeared significantly earlier than CFU-GM suggesting that early granulocytic recovery after bone marrow transplantation is mediated by proliferation of mature progenitors committed to the granulocytic lineage, whereas later reconstitution is accompanied by the emergence of CFU-GM.