Eventually, we undertook to combine PEG dialysis with ultracentrifugation. A wide-mouth funnel was inserted into the ultracentrifuge tube, and a cast cylinder of PEG was inserted into a dialysis tube that extended from almost the bottom of the ultracentrifuge tube to the top of the funnel. When a 100-ml sample was poured into the funnel, excess fluid was imbibed by the PEG inside the dialysis tube until the total volume could be contained by the ultracentrifuge tube. At this point, the funnel and the PEG-containing dialysis tube were removed, and the ultracentrifuge tube was capped and placed in the rotor (Herrmann and Cliver 1968b) . This approach was not compatible with use of the trap described above.