The contribution by potassium persulfate to suspension stability in the bead polymerization of styrene which is stabilized by hydroxyl apatite (tricalcium phosphate) powders has been studied. Potassium persulfate, when used in lieu of an anionic surfactant as the extender, causes the formation of polystyrene sulfate in the aqueous phase. This anion-active polymer adsorbs on the surface of the hydroxyl apatite crystals and thus alters their wetting characteristics, causing them to become more effective suspension stabilizers. Polystyrene sulfates of molecular weights ranging from 2000 to 100,000 have been synthesized, characterized, and shown to be effective hydroxyl apatite extenders. The capability of polystyrene sulfate to adsorb onto hydroxyl apatite crystals is not strongly dependent upon the molecular weight of the polymeric ester. The tolerance of the system to high molecular weight polystyrene sulfate is relatively high since excess extender is taken up by dissolving in the monomer phase. The tolerance for water-soluble polystyrene sulfate, on the other hand, is low because excess extender of this type engages in double-layer adsorption on hydroxyl apatite, which renders the latter too hydrophilic to permit effective suspension stabilization. Use of a water-soluble polymerization inhibitor in the suspension polymerization system has no effect on suspension stability when sodium dodecylbenzene sulfonate or preformed polystyrene sulfate is used as the extender; however, suspension stability is destroyed when the inhibitor is used with persulfate, because it prevents the formation of polystyrene sulfate. Potassium persulfate is therefore more accurately described as a precursor of the active extender, which is the mono- or disulfate ester of polystyrene.