Peripheral blood cells (PBCs) collected by leukapheresis after progenitor mobilization with chemotherapy and growth factors have been used successfully to replace marrow autografts in protocols requiring stem-cell support. Moreover, such transplants are often associated with more rapid recovery of blood cell counts than is routinely achieved with bone marrow. While conditions that mobilize colony-forming cells (CFCs) into the circulation are becoming increasingly well characterized, little information is available as to how these or other mobilizing treatments may influence the release of more primitive cells into the peripheral blood. To quantitate the peripheral blood content of such cells, we used the long-term culture-initiating cell (LTC-IC) assay, which detects a cell type that is able to produce progeny CFCs after a minimum of 5 weeks in cultures containing marrow fibroblasts. In this report, we present the findings on 21 patients who were transplanted over a 7-year period at our institution with PBCs alone. PBCs were collected in steady-state (n = 6) or during the recovery phase after high-dose cyclophosphamide (Cy; n = 15, nine with and six without additional growth factor administration). PBCs collected from another 11 patients given granulocyte colony-stimulating factor (G-CSF) were transplanted together with autologous marrow. Time-course studies of nine patients after Cy +/- granulocyte-macrophage CSF (GM-CSF) showed that CD34+ cells, CFCs, and LTC-ICs fell from normal to undetectable levels after Cy, and increased at the time of white blood cell (WBC) recovery: LTC-ICs to a mean of sixfold and CFCs to a mean of 26-fold higher than normal. The mean number of CD34+ cells, CFCs, and LTC-ICs present in the PBC harvest was twofold to 10-fold higher after mobilization than in steady-state collections; however, more than 2-log interpatient variability was observed. After PBC transplantation, the median time to a WBC count more than 10(9)/L was 12 days; polymorphonuclear leukocyte (PMN) count more than 0.5 x 10(9)/L, 15 days; and platelet count more than 20 x 10(9)/L, 17 days, although patients who received fewer than 1.5 x 10(5) CFCs/kg had a more than 50% chance of delayed count recovery (> 28 days). Patients who received Cy + GM-CSF-stimulated PBCs had more rapid and consistent platelet recoveries as compared with other groups receiving Cy mobilized or steady-state PBCs alone, and a rapid WBC recovery after Cy predicted a rapid WBC recovery after transplantation.