a filter system for separating and binding biomolecules contained in a solution includes a first filter means for selectively passing components of the solution that are smaller than the biomolecules , thereby concentrating the biomolecules in the solution . also included is a binding membrane means having an affinity for the biomolecules for binding the biomolecules to the binding membrane means , with the binding membrane means being located where the biomolecules are concentrated by the first filter means . in the preferred mode , the binding membrane means is chosen to selectively bind the biomolecule of interest . according to a preferred method of the invention , biomolecules from a sample volume are bound to a binding membrane by urging the sample volume against a biomolecule - binding support membrane that is in contact with a cut - off filter membrane such that the sample volume will pass through the biomolecule - binding support membrane before passing through the cut - off filter membrane . the concept is to concentrate the biomolecules of interest by means of the cut - off membrane in the vicinity of the binding membrane , thereby increasing the binding yield on the binding membrane . in the preferred mode , the sample volume is urged against the binding membrane by centrifugation .