Systematic studies are being conducted on the composition and solution properties of dextran-poly(ethylene glycol) two phase aqueous systems with a view to identifying those properties of the system which primarily determine the partition behavior of cells. The measurements which include the potential difference between bulk phases, salt partitioning, phase boundary surface potentials, interfacial tension and polymer concentrations are all aimed at providing an understanding of the interrelated factors which determine the cell partition coefficient. Cell separations by countercurrent distribution using dextran-PEG systems will be applied in attempts to segregate erythrocytes according to age and in the isolation of abnormal red cell populations. An example of the latter is paroxysmal nocturnal hemoglobinuria in which populations of abnormal red cells coexist with a normal red cell population. Adsorption of dextran and PEG to standard preparations of erythrocytes will be measured as well as the electrophoretic mobility of red cells suspended in the isolated phases. Subfractionation of other cell types, especially the blood cellular elements, will be attempted using the information obtained from the systematic studies on solution properties of the phases to test a thermodynamic model of cell partition which will be used in the development of more effective two-phase aqueous polymer systems for separation of cell subpopulations.