Methods, apparatus and compositions for separating a desired or undesired population or subpopulation from a biological sample are disclosed herein. The selection procedure is based on ferromagnetic, dense particles in a preferred size range from about 0.8 to about 1.2 microns. Specific binding agents are bound to the particles that recognize and bind to specific molecules on the targeted population or subpopulation, and the particles are mixed with the sample in such a way as to promote movement of the particles relative to the sample, promoting binding to the targeted population or subpopulation without non-specifically binding to non-targeted populations in the sample. Because of the large particle density, the bound population is separated from the fluid sample by gravity. Alternatively, the sample, including the bound, targeted population, is placed in a magnetic field such that the particles separate from the sample by evenly distributing over the vessel wall thus limiting non-specific trapping of the non-targeted population.