We review our recent experimental and theoretical work aimed at investigating the potential use of two-phase aqueous micellar systems for the separation or concentration of hydrophilic biomaterials using the principle of liquid-liquid extraction. The systems studied include (1) a two-phase aqueous micellar system composed of the nonionic surfactant n-decyl tetra(ethylene oxide) (C(10)E(4)) and (2) a two-phase aqueous micellar system composed of the zwitterionic surfactant dioctanoyl phosphatidylcholine (C-8-lecithin). The experimental partitioning behavior of several hydrophilic proteins, including cytochrome c, soybean trypsin inhibitor, ovalbumin, bovine serum albumin, and catalase, in two-phase aqueous C(10)E(4) and C-8-lecithin micellar systems is reviewed. A theoretical formulation of the protein partitioning behavior, based on a description of excluded-volume interactions between the hydrophilic proteins and the micelles, is also reviewed. The theoretically predicted protein partitioning behavior is compared with that observed experimentally and is found to be in good agreement. The results of our investigation suggest that two-phase aqueous micellar systems of the type examined in this article are indeed potentially useful as extractant phases for the separation or concentration of proteins and other biomaterials.