Circulating myeloid progenitor cells (PB CFU-GM) were measured in the peripheral blood of 13 patients with acute non-lymphoblastic leukaemia (ANLL) as they entered first remission. The mean of the recorded peak levels was 2796 X 10(3) CFU-GM/l, representing a 25-fold increase above the mean value in normal subjects. These elevated levels of PB CFU-GM occurred regularly during the very early remission phase when platelet counts rose rapidly. Five of the patients had PB mononuclear cells collected by continuous-flow leukapheresis during this early recovery phase. CFU-GM were assayed as a measure of the number of haemopoietic stem cells in each collection. The cells were concentrated and then cryopreserved in liquid nitrogen. Leukapheresis was also performed on five normal subjects for comparison. Low numbers of CFU-GM were harvested from normal subjects, mean 0.33 +/- 0.06 X 10(4) CFU-GM/kg body weight for each leukapheresis. In ANLL patients entering remission, however, very large numbers of CFU-GM were regularly harvested. A mean of 11 +/- 2 X 10(4) CFU-GM/kg body weight were cryopreserved after each leukapheresis, representing 5 times the number of CFU-GM considered necessary for successful autologous haemopoietic reconstitution. Haemopoietic stem cell viability was assessed after varying periods of cryopreservation. There was no significant stem cell loss after up to 24 months storage. Thus, it is possible to collect and cryopreserve large numbers of CFU-GM and by inference pluripotent haemopoietic stem cells from the peripheral blood of patients with ANLL during very early remission. The possible biological and therapeutic implications are discussed.