A sol-gel procedure was used to cover the magnetic P (St-co-MAA-co-AM) microspheres with a SiO2 layer, forming MMS/SiO2 composite micospheres. The composite spheres were synthesized by a two-step process. First, magnetic P (St-co-MAA-co-MA) microspheres (MMS) were prepared by inicroemulsion polymerization of styrene (St), methacrylic a,id (MAA) and acryamide (AM) in the presence of magnetic fluid. In the next step, MMS/ SiO2 spheres were obtained by addition of tetraethyl orthsilicate (TEOS) to the mixture of MMS, ammonium hydroxide and ethanol. The morphology, structure and properties of the MMS/SiO2 were characterized and the results indicate that the MMS/SiO2 spheres are about 2 pm in size with superparamagnetic behavior. The surface of the MMS/SiO2 spheres was functioned with -COOH successfully. Bioconjugation to IgG was also studied.