The median tumour cell reductions in log steps were 3.7 (2.9-4.3) (n ‫؍‬ 13) (Isolex-50), 3.5 (2.6-4.3) (n ‫؍‬ 13) (MiniMACS) and 1.5 (0.9-2.9) (n ‫؍‬ 17) (Ceprate-LC). Results were compared statistically by univariate analysis. Purity was significantly (P Ͻ 0.05) better after Mini-MACS selection. Recovery rates were significantly different between all devices tested. Tumour cell purging was superior after immunomagnetic separation (P Ͻ 0.001). Tumour cell purging is a main objective of CD34 + selection in the autologous setting. High-dose chemotherapy supported by autologous stem cell reinfusion for treatment of metastatic and high-risk female breast cancer is currently under investigation in several American and European studies. 1,2,3 Tumour cell contamination of autologous stem cell products has been described using immunocytochemistry, reverse transcriptase PCR and cell culture techniques in approximately one third of harvests with a range from 0% to 100%. [4][5][6][7] The clonogenic and metastatic potential of tumour cells isolated from blood and autografts was demonstrated in cell culture assays and in nude mice. [8][9][10] The metastatic potential of accidentally retransplanted tumour cells could be investigated by gene marking of autografts prior to infusion followed by the detection of marked cells after relapse. 11 However, due to the poor transduction efficacy of epithelial cancer cells by gene transfer, a negative result would not exclude the possibility of relapse induction by accidentally reinfused tumour cells. Different approaches have been described to purge autografts of tumour cells. 12 A major problem of purging procedures using cytotoxic agents is the damage to haemopoietic progenitor cells with subsequent delay of engraftment, an increased incidence of severe infections due to prolonged neutropenia and bleeding complications due to thrombocytopenia. [13][14][15] Haemopoietic cells necessary for recovery after highdose therapy and progenitor support express the CD34 antigen on their surface. 16 These cells can easily be enriched by immunological methods using anti-CD34 antibodies and separation by biotin-streptavidin immunoaffinity or magnetic separation with paramagnetic microbeads. Rapid and safe engraftment after reinfusion of enriched CD34 + cell fractions has been described by several investigators after allogeneic and autologous transplantation. 17,18 The immunologic selection of CD34-positive cells has no toxic side-effects on haemopoietic stem cells and progenitors without prolongation of cytopenia after reinfusion.Furthermore, CD34 positive-cell selection is a promising approach to reducing the incidence or mitigating the severity of graft-versus-host-disease in the allogeneic setting by adjusted or nearly complete T cell depletion. However, the major rationale for CD34-positive cell selection in autologous marrow and stem cell transplantation is a reduction or removal of contaminating tumour cells accidentally coharvested during leukapheresis to avoid their reinfusion after high-d...