Soybean agglutinin (SBA) was used as a differential reagent to achieve selective elimination of human breast cancer cells (T-47D cell line) from human marrow contaminated with tumor cells. Two successive cycles of direct agglutination by soluble SBA resulted in depletion of 3.5 logs of tumor cells as determined by radiolabeling, whereas removal of more than 4 logs of tumor cells was demonstrated by a clonogenic bioassay. A more convenient procedure for tumor purge involved the use of SBA bound to either polyglutaraldehyde magnetic beads or to commercial polystyrene magnetic beads. After one cycle of magnetic separation, 2 to 3.5 logs of tumor cells were removed. A second separation cycle using fresh magnetic beads improved depletion to more than 4 logs. Neither of these purging procedures affected the hematopoietic potential of granuloid-macrophage colony-forming unit cells. We suggest the use of SBA bound to magnetic beads as a convenient tool for effective ex vivo purging of marrow aspirates contaminated with metastatic breast cancer cells in patients with advanced disease. A similar procedure is applicable for all SBA-positive neoplasms.