The extraction of bovine serum albumin (BSA, M. W. 66 kDa, pI 4.9) was carried out using a reverse micelle consisting of sodium bis(2-ethylhexyl) sulfosuccinate (AOT). When CaCl2 or MgCl2 was used, BSA could be extracted at a pH higher than the pi of BSA. This effective extraction at a pH higher than the pi was considered to be caused by the modification of interactions between the protein and the micelles by the divalent cations. For KCl and NaCl, BSA could be extracted only at high concentrations of AOT and within a narrow pH range, and maximum extraction was achieved at the pH values of 5.5 for NaCl and 5.0 for KCl. The BSA extraction efficiency decreased sharply at high concentrations of NaCl and KCl. Sufficient concentrations of AOT in the organic phase, which depends on the salt type and its concentration, are required to extract BSA into the organic phase. In the range of insufficient concentration of AOT, aggregation and precipitation of BSA with AOT occurred in preference to extraction. BSA extracted into the organic phase could be back-extracted into a fresh aqueous phase at high pH values and high salt concentrations. The conformation of BSA was maintained to a large extent during the extraction process.