The levels of circulating hematopoietic progenitor cells were measured sequentially in eight children receiving chemotherapy for acute leukemia or neuroblastoma. Significant increases in the progenitor levels (up to 50-fold in CFU-GM numbers) were observed during post-chemotherapy cytopenia in all cases, but differences among individuals in the kinetics of recovery of less committed progenitors (CFU-mix) contrasted with the synchronized-mode of expansion of committed progenitors (CFU-GM). Peripheral blood cells were collected by repeated continuous-flow leukaphereses from three of the children during post-chemotherapy expansion of the progenitor pool and were cryopreserved after fractionation procedures. Infusion of these stored cells into the patients after marrow-ablative chemotherapy established trilineage hematopoiesis. This use of stem cell rescue should be useful as an alternative to bone marrow transplantation and extends the application of cure-oriented salvage therapy to childhood cancers.