The partitioning behaviour of an alkaline protease from a Bacillus sp. was studied in aqueous two-phase systems comprising polyethylene glycol (PEG-6000) and K2HPO4. Based on the binodal curve various combinations of PEG-6000 and K2HPO4 were employed for enzyme partitioning and one with 5% (w/v) PEG-6000 and 35% (w/v) K2HPO4 was found most effective. The enzyme activity was substantially stimulated by PEG-400, PEG-6000 and K2HPO4. Copyright (C) 1996 Elsevier Science Ltd.