Most membrane functions are mediated by enzymes and specific receptors that bind small molecules: substrates, inhibitors, modulators, hormones, and neurotransmitters. At the molecular level, polypeptide chains that contribute to the binding site of a given receptor can be identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) if they are specifically labeled with a radioactive marker. Purification and subsequent N-terminal sequencing of these labeled peptides identify their location in the primary sequence of the receptor. Photoaffinity labeling has emerged as an effective tool to tag amino acid residues covalently that are in or near ligand binding sites (1). The specificity of this method arises from the following characteristics.