A novel approach for regioselective synthesis is described. The approach uses the unique binding properties of protein as a guide to achieve regioselectivity. Specifically, steroids, e.g., 4-androstene-3,17-dione, were reduced selectively at carbon 17 using bovine serum albumin (BSA) as a regio auxlliary. The approach has resulted in a high yield (ca. 91%) in an environmental friendly aqueous medium. Both H-1 NMR and C-13 NMR data show that the BSA-assisted reduction of the steroid render the reaction regioselective compared to the control reaction. Various control reactions were performed to verify that the unique binding of BSA to steroids was the origin of regioselectivity. After one cycle of the reduction reaction, BSA was reconditioned such that it could be reused five or more times. This novel approach could be used to develop practical and "green" methods for regioselective synthesis. (C) 2003 Wiley Periodicals, Inc.