Peripheral blood progenitor cells (PBPCs) are being used increasingly to provide hematopoietic support after intensive chemotherapy. However, many investigators have detected tumor cells contaminating PBPC collections. Methods that eliminate the tumor cells and spare the normal hematopoietic progenitor cells may improve the number of long-term, disease-free survivors after intensive chemotherapy. We developed an effective method using anti-breast cancer murine monoclonal antibodies (MoAbs) and immunomagnetic beads to eliminate a low percentage of breast cancer cells from PBPCs. We identified optimal anti-breast cancer MoAbs that react with membrane glycoproteins and conditions for selective removal of tumor cells. Using three anti-breast cancer MoAbs (260F9, 317G5, and 520C9) at 0.8 microgram/ml, a cell concentration of 2 x 10(8) cells/ml and a bead:total cell ratio of 0.75 beads:1 cell, we eliminated 3.3-4.8 (mean, 4.1) logs of tumor cells consistently from a model system with 1% breast cancer cells and 99% normal PBPCs. Similar levels of tumor cell elimination were obtained with three breast cancer cell lines. Colony-forming units were not affected adversely, with a mean recovery of 200% compared with the control. A clinical trial has begun that uses immunomagnetically purged, autologous bone marrow and PBPCs to support patients with metastatic breast cancer receiving high-dose chemotherapy.