L-5-hydroxytryptophan (5-HTP) with two types of multiple F-19-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 F-19 magnetic resonance spectroscopy studies. The enzymatic assay described here provides an efficient and cost-effective tool for screening new compounds.