A composition comprising colloidal Fe.sub.3O.sub.4 particles coated with
streptavidin is disclosed. These particles can be used in a method for
separating biotinylated compounds from a solution in which the
biotinylated compounds bind to the streptavidin-coated particles which
are attracted to a surface by a magnet. Also disclosed is a method for
making said streptavidin-coated composition, said method comprising the
steps of forming colloidal Fe.sub.3O.sub.4 particles by mixing aqueous
FeCl.sub.2 with aqueous FeCl.sub.3, adding aliquots of the mixture to an
alkaline solution and adding streptavidin.