Summary:High-dose chemotherapy (HDC) with hematopoietic support appears promising in the treatment of breast cancer, although reinfusion of contaminating tumor cells may contribute to disease relapse. Ex vivo expansion may reduce tumor cell content through use of a small inoculum volume and by passive purging during culture. We assessed the ex vivo expansion potential of tumor cell positive bone marrow (BM) from breast cancer patients and the effect of ex vivo expansion on tumor cell content. Cryopreserved/thawed mononuclear cell (C/T MNC) BM harvests with known tumor cell contamination (n = 7) were assessed for tumor cells preand post-expansion using immunocytochemical (ICC) staining. Pre-expansion inoculum samples contained a range of 6-2128 tumor cells per 5.0 × 10 6 nucleated cells. Ex vivo expansion resulted in fold expansions of 6.67 and 11.37 for total cells and CFU-GM, respectively. Tumor cells were undetectable in four of the seven postexpansion samples and were reduced in the remaining three samples. The data demonstrate passive purging of breast cancer cells during ex vivo expansion, with hematopoietic progenitor cell expansion comparable to that of normal BM. Reduction in tumor cell number contained in the small volume culture inoculum combined with passive purging during the ex vivo expansion process suggest a potential 2-4+ log reduction in tumor cell content in the reinfused cell product. Keywords: bone marrow transplantation; ex vivo expansion; tumor cell contamination Immunologic techniques have revealed tumor cell infiltration of the BM in as many as 48% of early stage breast cancer patients at the time of diagnosis. [1][2][3][4] The presence of tumor cells in the BM has been shown to be predictive of relapse in early stage 5-7 and advanced stage 8,9 breast cancer patients. In addition, the number of tumor cells identified in the BM has been correlated with shorter disease-free and overall survival. 6 Although the prognostic significance of tumor cells in PBPC collections requires further evaluation,