Fresh human bone marrow (BM) mononuclear cells containing a mean of 11% T cells underwent T cell depletion utilizing a newly developed magnetic cell sorter. In order to remove T cells, 10(8) cells were labeled with either a specific CD3 monoclonal antibody or a cocktail of monoclonal CD2 + CD5 + CD7 antibodies and biotin-conjugated magnetic microparticles (size 100-150 nm) employing a streptavidin-FITC complex. Labeled cells were removed by using columns filled with a ferromagnetic matrix and placed within a recess in a strong permanent magnet. Recovery of non-T cells was 79% after the first passage and 61% after the second. The absolute recovery of BFU-E and CFU-GM was 70% and 51% respectively (n = 11). Limiting dilution analysis revealed less than 0.1% residual clonable BM T cells (cloning efficiency greater than 50%) which corresponds to a 2.1 (1st passage) and to a 2.3 (2nd passage) log depletion. This new separating system is a highly efficient, reproducible, and quick method of depleting T cells from human BM. It could be easily adapted to process large scale marrow specimens or for the elimination of residual tumor cells from BM in autologous transplantation.