l-5-hydroxytryptophan (5-HTP) with two types of multiple (19)F-atom tags bonded at various positions onto the indole ring (positions 4, 6, or 7) was exposed to aromatic l-amino acid decarboxylase (AADC) in lysates of Escherichia coli JM109 which had been transformed with the plasmid pKKAADCII. Resulting samples were analyzed with HPLC. In the first study, which investigated a straight-chain seven-atom tag, a novel peak, putatively perfluoro-tagged serotonin, was detected. A second study demonstrated that 5-HTP was converted to 5-HT in transformed E.coli lysate but not in untransformed lysate. A third study, investigating a tag with nine fluorine atoms all in the same nuclear environment, identified the isomer serving as the best substrate for AADC. This novel molecule had the tag bonded at the 6 position on the indole ring. Isomers that fit into the active site of AADC are likely to follow the biosynthetic path for serotonin in vivo and are potentially useful in (19)F magnetic resonance spectroscopy studies. The enzymatic assay described here provides an efficient and cost-effective tool for screening new compounds.