We introduce a new concept for the purification and concentration of macromolecules such as proteins based on the use of magnetic adsorbents in concert with micellar aqueous two-phase systems. Aqueous solutions of certain nonionic surfactants can, depending on the prevailing temperature, separate into two co-existing phases, one enriched, the other depleted in surfactant. These are known as micellar aqueous two-phase systems. Hydrophilic proteins are normally excluded from the 'surfactant-rich' phase of micellar aqueous two-phase systems. We show here that by introducing magnetic microadsorbents into micellar aqueous two-phases the phase selectivity of a hydrophilic protein can be effectively inverted. The magnetic adsorbents adsorb the target protein and pull it out of the 'surfactant-depleted' phase into the 'surfactant-rich' phase from which it is normally excluded. Starting from a model solution containing a 1:1 ratio of the hen egg white proteins lysozyme and ovalbumin, the application of magnetic cation exchange micro-adsorbents combined with the nonionic surfactant Triton X-114 resulted in a target lysozyme yield of 74% and purity of >80%. The combination of selective magnetic adsorbents with micellar aqueous two-phase systems yields a highly tunable extraction system. Potentially, these so-called 'Magnetic Extraction Phases' provide the basis for flexible and easily scaled fractionation processes for hydrophilic proteins. (C) 2008 Elsevier B.V. All rights reserved.