Abstract: Magnetizable composite microspheres comprising a core of a magnetizable filler distributed uniformly in a network of a polysilsesquioxane SiVi containing double bonds and a shell of a crosslinked organosilicon polymer derived from the hydrosilylation of an organohydrogenopolysiloxane SiH with the ethylenically unsaturated groups of the polysilsesquioxane. The magnetizable composite microspheres are obtained by dispersing an aqueous suspension of a magnetizable filler, not coated with a dispersing agent, in a solvent, dissolving an alkoxysilane or an alkoxysiloxane containing double bonds, polycondensing to a SiVi group-containing polysilsesquioxane SiVi, removing water, dissolving the organohydrogenopolysiloxane SiH polymer in the organic phase, crosslinking the mixture of polymers, removing water, separating the microspheres and, if appropriate, redispersing the microspheres in water. The magnetizable composite microspheres are preferably used as an active supports in biology.