Ex vivo culture of hematopoietic progenitor cells for autologous transplantation has generated world-wide interest, since it offers the prospect of using a limited cell number, and may allow more efficient gene transfer and passive elimination of contaminating tumor cells. In this study, we expanded bone marrow (BM) cells from 10 breast cancer patients to determine whether small BM aliquots can durably restore hematopoiesis, and whether thrombopoietin (TPO) improves hematopoietic reconstitution after myeloablative chemotherapy. We used the AastromReplicell System (ARS), performing a computer-controlled, stromal-based cell expansion process with frequent medium, cytokine and gas exchange. For the inoculation of 9 x 10(8) MNC, a median BM volume of 97.8 ml (range, 72.4-272) was harvested. We found a median 4.5-fold nucleated cell expansion, an 18-fold CFU-GM expansion, and 69% of input LTC-IC numbers. Nucleated and Lin-/CD34+ cells were infused with a median of 43.5 x 10(6)/kg (range, 34.1-71.7) and 2.8 x 10(5)/kg (range, 0.95-5.9), respectively. Despite tumor cell detection by immunocytochemical staining in 3/10 patients before expansion, tumor cells were not detectable in 9/10, and in one patient 1 log reduced post ARS culture. Following high-dose STAMP V chemotherapy, all patients received 12-day expanded BM cells. The median time to engraftment was 17 days (range, 11-20) for WBC >1000/microl, and 28 days (range, 21-55) for platelets >20,000/microl. A correlation between post-expansion Lin-/CD34+ cells and engraftment for ANC >500/microl, WBC >1000/microl and platelets >20,000/microl was observed. Hematopoiesis has been maintained for a median of 15 (range, 6-24) months. Our results demonstrate that transplantation of ex vivo expanded small BM aliquots allows hematopoietic reconstitution after myeloablative chemotherapy. Ex vivo generated ARS cells can reduce the risk of tumor cell reinoculation with autotransplants and may be valuable in settings in which only small stem cell doses are available, eg when using cord blood transplants or in non-mobilizing patients.