In protein extraction, using the nonionic surfactant sugar ester DK-F-110, the critical micelle concentration (CMC) of a DK-F-110/isopropyl alcohol(IPA)/hexane system was found to be at a DK-F-110 concentration of 0.5 g dm(-3), indicating that the sugar ester reverse micelles could be formed in hexane. At concentrations higher than the CMC, cytochrome c was extracted into the DK-F-110 reverse micelles as judged from UV spectra of the DK-F-110/IPA/hexane solution after it was contacted with the aqueous protein solution. In extraction of cytochrome c using this system, forward extraction was found to be pH-dependent, with high extraction percentage being obtained at pH8. The forward extraction percentage was reduced by an increase in the buffer concentration, and at a buffer concentration of 0.5 mol dm(-3) was ca 25% as high as that of sodium bis(2-ethylhexyl) sulfosuccinate(AOT) systems. An optimal DK-F-110 concentration was found to give the maximum forward extraction percentage. Addition of alcohol, especially IPA, to the micellar organic phase enabled the highly backward extraction of cytochrome c to be achieved without the formation of insoluble aggregates. The esterification reaction rate by Rhizopus delemar lipase in the DK-F-110 reverse micellar system had a higher maximum value than that of AOT and lecithin systems.