The design of aqueous two-phase systems (ATPSs) which support the long-term growth of animal cells is described in this paper. It was found that the increase in osmolality caused by the ATPS-forming polymers could be compensated by reducing the NaCl concentration of the culture medium. Cell growth was possible in culture media containing up to 0.025 g g(-1) PEG or 0.15 g g(-1) dextran. In ATPSs of PEG 35,000; dextran 40,000; and culture medium; the hybridoma cells partitioned to the PEG-lean phase. In two of these ATPSs, hybridoma cells were successfully cultured over a period of more than two months. The Mab product, however, partitioned along with the cells in the lower phase, but preliminary experiments using PEG ligands showed improved Mab partitioning.