Water-in-oil (w/o) microemulsion of sucrose fatty acid ester was used to renature denatured hen egg white lysozyme without aggregation. After lysozyme was denatured in 5 M guanidine hydrochloride for 24 h, the resultant denatured lysozyme was held in the microemulsion, overnight at 25 degrees C. Renatured lysozyme was transferred from the microemulsion phase to the recovery aqueous phase by conventional liquid-liquid extraction. The enzymatic activity of the recovered lysozyme was 93%.