Breast cancer is the most common female cancer. Its incidence is increasing rapidly in younger women, where early detection is infrequent, tumors tend to be more aggressive, and loss of productive years is greatest. High-dose chemotherapy with autologous hematopoietic progenitor cell support (AHPCS) has demonstrated effectiveness for poor-prognosis breast cancer patients. The focus of this ROl is the evaluation of AHPCS using CD34+ hematopoietic progenitor cells, which are then subjected to further manipulation(s). The goals of this project are to: l. Maximally decrease the amount of breast cancer in the autograft. 2. To produce rapid and durable marrow reconstitution in breast cancer patients receiving HDC. 3. To reduce the toxicity of infusing large volumes of unpurified AHPCS by using purified progenitor cells. 4. To develop ex vivo expansion of hematopoietic progenitors as a novel technology, and demonstrate its clinical utility.