The recovery of proteins from animal blood using an established two-stage extraction process was selected as a practical model system to study the generic application of polyethylene glycol (PEG)-phosphate aqueous two-phase systems (ATPS). Processing of whole bovine blood in the ATPS two-stage process resulted in the partition of soluble protein (e.g. bovine serum albumin (BSA), haemoglobin, IgG; partition coefficient K = 55) into a PEG-rich top phase and cell debris into a phosphate-rich bottom phase. Subsequent back extraction of soluble protein into a second phosphate-rich bottom phase resulted in a maximum overall protein recovery of 62%. The increased protein concentration within the ATPS (from 1.2 to 7.0 mg/g) caused a decreased in the recovery to 44%. Recycling of PEG into the initial extraction stage did not significantly influence the partition behaviour of protein over the equivalent of 20 operational cycles, but protein recovery decreased from 44 to 37%. The extreme conditions (waste material characterised by the presence of solids and impurities) in which the implementation of this ATPS process was tested, confirms the potential for the generic application of ATPS for processing complex biological suspensions to achieve a simple primary recovery and partial purification of target protein solutes. (C) 2000 Elsevier Science Ltd. All rights reserved.