The partitioning of heterogeneous cell populations of rat bone marrow, in comparison with that of homogeneous populations of erythrocytes, has been studied in aqueous two-phase systems containing increasing concentrations of dextran-poly-(ethylene glycol) (to increase the interfacial tension) and/or decreasing phosphate concentrations (to decrease both the electrical potential difference between the phases and the interfacial tension). At any polymer and/or phosphate concentration tested the partition is lower for bone marrow cells, reflecting their lower surface charge and hydrophobicity.