An improved method for separating materials is provided, using colloidal, magnetizable aggregates, optionally silanized, and coated with a one or more layers of novel polysaccharide derivatives. Materials separated by the aggregates of the invention include inorganic and organic molecules, viruses, organelles, and cells. The invention also relates to a kit for separating such materials. The separated materials are useful in analytical and preparative or in diagnostic and therapeutic techniques.