Selective elimination of multidrug resistance-positive cells (LoVo/Dx) was obtained by using the monoclonal antibody MRK 16, which recognizes a surface epitope of the Mr 170,000 glycoprotein, and a sheep anti-mouse immunoglobulin antibody, conjugated to the ribosome-inactivating protein saporin 6. The killing was greatly decreased or even abolished by adding the monoclonal antibody at a 100-fold concentration. Both the MRK 16 and anti-mouse saporin 6 conjugate did not show any killing activity when they were used separately. In cell suspensions composed of 90% normal bone marrow cells and 10% multidrug resistance-positive cells, the monoclonal antibody MRK 16 followed by the anti-mouse immunotoxin caused the elimination of 99% multidrug resistance-positive cells, as revealed by immunofluorescence and immunocytochemistry as well as by a clonal assay. Human normal hematopoietic precursors (granulomonocytic colony-forming units, erythroid burst-forming units, and multipotent granulomonocytic, erythroid, and megakaryocytic-forming units) were not affected by the MRK 16 plus immunotoxin treatment. This technique might be suitable for ex vivo bone purging in an appropriate clinical setting, such as autologous bone marrow transplantation.