Sorting heterogeneous populations of cells into specific subpopulations has been greatly facilitated by the advent of monoclonal antibodies (MAbs) in combination with magnetic particles (1 –35 ). MAbs provide the specificity necessary to distinguish one cellular subpopulation from another, and the magnetic particles provide the physical force for moving the target population away from the remaining cells. Two forms of selection have been used by researchers: positive and negative selection. Positive selection is the isolation of the desired population from the cellular mixture. The methods for positive selection are described in Chapter 26 . Negative selection is the elimination of an undesired population from the cellular mixture. In this chapter, we focus on negative-selection methodology (1 –8 ,10 –27 ,29 –35 ).