Protein solubilization was studied for two kinds of reversed micellar systems with the aid of two phase transfer methods. The first system was composed of nonionic surfactant, poly(oxyethylene) alkyl ether (C(i)E(j)), and sodium bis(2-ethylhexyl)sulfosuccinate (AOT). Proteins used were a-chymotrypsinogen A (ctn) and cytochrome-c (cytc). In the absence of AOT, protein was solubilized in the micelles by the entropic effect. Then, the fraction E of the proteins extracted to the micellar phase was very low. E increased with the addition of AOT due to the electrostatic interaction between AOT-anions and proteins. The denaturation of proteins in the micelles was restrained by controlling the electrostatic interaction. The solubilization mechanism was simplified by using this extraction system. The second system was composed of C(i)E(j) and alkylglucopyranoside (AG(n)), which worked as an amphiphilic affinity ligand to concanavalin A (conA). ConA was not extracted to the reversed micelles consisting of C(i)E(j) alone. With the addition of AG(n), conA was taken up to the reversed micelles through the biospecific interaction. The reversed micelles tailored by controlling the interactions with proteins provide an ideal extraction system for proteins.