Low tumor-associated antigen (TAA) expressing tumor cells present an obstacle to effective antibody directed purging of tumor cells from bone marrow. In this study, a comparison was made of the efficiency with which low TAA expressing leukemia cells could be depleted using two monoclonal antibody (MoAb) directed purging techniques: 1) complement (C)-mediated cytolysis, and 2) physical separation using magnetic microspheres. Low TAA sublines were selected from a cultured human leukemia cell line by growing out cells remaining after treatment with anti-TAA and C, or after immunomagnetic (IM) purging. IM-selected sublines showed lower TAA expression than did C-selected sublines, and sublines resulting from multiple selections expressed less TAA than those that had only been through one selection. These sublines were then examined for sensitivity to C or IM purging. The highly selected, lowest TAA expressing sublines were markedly resistant to both IM and C. Less selected sublines were resistant to C, but not to IM. In both techniques, addition of MoAbs against a second TAA restored the efficiency of purging to that observed with the parental line. When low TAA subline cells were seeded into simulated bone marrow and subjected to purging, C-mediated lysis removed less than 40% of leukemia cells, whereas IM purging removed 85% of the cells. These results indicate that there are low antigen density cells that are resistant to C-mediated purging, but which retain sensitivity to IM removal.