Polyampholytic acrylic co-polymers have been shown to form two-phase systems with polyvinyl alcohol under appropriate conditions of pH, ionic strength and temperature. These novel systems have been used for the purification of a number of proteins from crude extracts. The partition behavior of proteins appeared to be influenced predominantly by electrostatic interactions and could be manipulated by including electrolytes in the system. Human serum albumin could be purified to apparent homogeneity (2.2-fold) in a single extraction step from plasma whilst partial purifications of trypsin (3.4-fold) and carboxypeptidase G2 (3.5-fold) were also obtained. In contrast to conventional PEG/dextran two-phase systems, quantitative protein recovery from liquid phases containing polyampholytes could be achieved simply by iso-electric precipitation of the polymer. © 1988.