cross-flow diafiltration was used for the recovery of $beta$ -galactosidase from a poly (ethylene glycol) (peg) 4000 top phase, which was obtained during extraction of escherichia coli cell disintegrate in a peg 4000/potassium phosphate aqueous two-phase system. two different filtration systems were used, an amicon system containing a hollow fiber filter (nominal cut off 100,000, membrane area 2.65 $center dot$ 10** $minus$ **2 m**2) and a millipore system containing a plate and frame filter (nominal cutoff 100,000, membrane area 0.5 m**2). to enhance filtration, the top phases were diluted 2.5 $multiplied by$ , 5 $multiplied by$ , and 10 $multiplied by$ with water prior to the filtration. by reducing the concentration of top phase before the filtration, the retention of $beta$ -galactosidase and peg 4000 decreased and the flux increased in both systems. while the retention of $beta$ -galactosidase was high in both systems, the retention of peg 4000 was much lower in the amicon system. however, the system capacities, expressed as the volume of nondiluted top phase treated per membrane area and time, were not influenced by the extent of dilution of the top phase prior to the filtration. (edited author abstract) 22 refs