The effect of 5-fluorouracil (5-FU) pretreatment on human bone marrow (BM) progenitor/stem cells and recovery of hematopoiesis after autologous marrow transplant was studied. Twenty-one patients were treated with 5-FU (15 mg/kg to 45 mg/kg) intravenously (IV) for 1 to 3 days administered 6 to 22 days before BM harvest. Post-FU marrow was infused into 15 patients after high-dose cyclophosphamide, carmustine (BCNU), and VP-16 (CBV). Seventeen patients (historical controls) were treated with CBV and autologous BM transplantation but did not receive 5-FU before marrow harvest. The groups were comparable for diagnosis and prior therapy. In the 5-FU-treated group and control group, median recovery times for platelet count to 50,000/mm3 were 20 and 30 days, respectively (P = .007), and for platelet count to 100,000/mm3, 23 and 38 days, respectively (P = .007), while neutrophil recovery was not significantly altered. In vitro cultures with 1 to 7 growth factors (interleukin-1 [IL-1], IL-3, IL-4, IL-6, colony-stimulating factor-1 [CSF-1], granulocyte-macrophage colony-stimulating factor [GM-CSF], and G-CSF) were performed. In 8 of 10 patients whose marrow was studied before and after 5-FU treatment, the numbers of CFU-C responsive to the combination of GM-CSF and IL-3 was increased 6.15-fold by 5-FU pretreatment. In 4 of these patients, thymidine suicide of GM-CSF- and IL-3-stimulated CFU-C ranged from 17% to 42%. High proliferative potential colony-forming cell (HPP-CFC) was observed in low frequency in normal marrow and patient's marrow before 5-FU treatment. In 11 of 16 patients pretreated with 5-FU, increased numbers of HPP-CFC were noted. GM-CSF and IL-3 interacted synergistically to stimulate HPP-CFC. Multifactor combinations, especially GM-CSF + G-CSF + IL-3 + IL-6 + IL- 1 + CSF-1 did not increase total colony count or classic HPP-CFC but did result in altered morphology, producing huge, loose colonies. The marrow from patients pretreated with 5-FU is enriched with multifactor- responsive HPP-CFC, renews in vivo granulopoiesis in a manner comparable with marrow harvests without 5-FU pretreatment, and provides accelerated in vivo platelet recovery. This marrow may be an appropriate target marrow for gene insertion in gene-therapy protocols.