Peripheral blood mononuclear cells (PBMC) were collected after the administration of recombinant human granulocyte colony-stimulating factor (rhG-CSF) and used as the sole source of hematopoietic stem cells after myeloablative therapy with busulfan (Bu) and cyclophosphamide (Cy). These studies were performed in 12 patients with malignancies (4 non-Hodgkin's lymphoma, 5 breast cancer, 1 testicular carcinoma, 1 Wilm's tumor, and 1 undifferentiated carcinoma) who had bone or bone marrow disease or had low marrow cellularity. rhG-CSF (16 micrograms/kg/d) was administered for 5 to 7 days by subcutaneous injection and PBMC were collected for 2 to 5 days beginning on day 4 after initiation of rhG-CSF, using continuous-flow blood-cell separators that processed 10 to 12 L of whole blood. From a median of three collections, a mean of 24.0 x 10(8) (+/- 10.5 SD) total nucleated cells/kg containing 12.6 x 10(8) (+/- 4.5 SD) mononuclear cells/kg, 7.3 x 10(6) (+/- 4.3 SD) CD34+ cells/kg and 20.5 x 10(4) (+/- 28.1 SD) granulocyte-macrophage colony-forming units (CFU-GM)/kg were harvested and cryopreserved. After the administration of Bu 14 to 17 mg/kg and Cy 120 to 150 mg/kg, PBMC were thawed and infused. One patient received rhG-CSF after the infusion of PBMC and the remaining 11 patients did not receive postinfusion growth factors. Mean days to recovery of neutrophil levels of 0.1, 0.5, and 1.0 x 10(9)/L were 11.4 (range, 9 to 13), 12.7 (range, 10 to 15), and 13.6 (range, 11 to 16) and the mean day to platelet transfusion independence was 13.3 (range, 7 to 49). Time to recovery of neutrophils to 0.5 and 1.0 x 10(9)/L and platelets to 20 x 10(9)/L was more rapid than in historical patients treated with Bu and Cy who received marrow alone or marrow followed by the posttransplant administration of rh-G or GM-CSF. No graft failures have been observed with a follow-up of 4 to 12 months. These results indicate that PBMC collected after rhG-CSF lead to rapid hematopoietic recovery after myeloablative chemotherapy.