The extraction of lysozyme, alpha-chymotrypsin, and pepsin from buffered salt solutions into reverse micelles was examined at different pH values and surfactant concentrations. The reverse micelles were formed by mixing aqueous buffer supplemented with KCl and an organic phase of isooctane(2,2,4-trimethylpentane), containing the anionic surfactant, Aerosol O. T. (dioctyl ester of sodium sulfosuccinic acid). The technique of dynamic laser scattering was used to measure the size of reverse micelles which were in equilibrium with the aqueous phase. It was found that the size of the reverse micelles decreased with increasing ionic strength but increased with increasing AOT concentration. In the process of extraction the reverse micelles might have rearranged themselves to host the protein. The sizes of protein-filled and -unfilled reverse micelles were different and an open equilibrium could be reached between them. Under the extraction conditions, only a small number of micelles were found to contain protein.