The amphiphilic biodegradable polymer surfactant, poly(ethyleneoxide monooleate)-block- poly(D,L-lactide) (MOPEO-PLA), was shown to form microemulsions in chloroform solution and to extract water and protein into their microemulsions from the aqueous phase. The water content in the chloroform phase significantly increased with the MOPEO-PLA concentration, indicating that a MOPEO-PLA microemulsion was formed with a large amount of water molecules. The largest amount of cytochrome c in the MOPEO-PLA microemulsion phase was extracted when the pH of the aqueous phase was close to the pI value for cytochrome c, and the degree of extraction increased at higher temperatures. From these results, the main factor for the extraction of cytochrome c was found to be the hydrophobic interaction between MOPEO-PLA and cytochrome c. The MOPEO-PLA microemulsion provides a benign extraction process for proteins. A MOPEO-PLA microemulsion system would provide a new encapsulation system in a biodegradable polymer matrix for delivery of proteins and peptides.