biocompatible polymeric microspheres having an average diameter below about 3 microns and having a density at least 15 % greater or lesser than organic cells and having covalent binding sites are provided in accordance with this invention . the microspheres are obtained by copolymerizing a hydroxy or amine substituted acrylic monomer such as hydroxyethylmethacrylate with a light or dense comonomer such as a fluoromonomer . a lectin or antibody is bound to the hydroxy or amine site of the bead to provide cell specificity . when added to a cell suspension the marked bead will specifically label the cell membrane by binding to specific receptor sites thereon . the labelled membrane can then be separated by density gradient centrifugation .