A new label transfer method is presented that overcomes most of the limitations of current systems. A protein of interest is tagged with a tetracysteine sequence (FlAsH receptor peptide (FRP)) that binds tightly and specifically to a chimeric molecule 3,4dihydroxyphenylalanine-biotin-4',5'-bis(1,3,2-dithioarsolan-2-yl)fluo rescein (DOPA-biotin-FlAsH). Upon brief periodate oxidation, the DOPA moiety is cross-linked to nearby surface-exposed nucleophiles. Boiling the products in excess dithiol dissolves the FlAsH-FRP interaction, resulting in transfer of the biotin tag to the partner proteins, allowing them to be identified by standard methods.