A method was developed to study amino acids as possible precursors of PQQ biosynthesis. Cultures of Hyphomicrobium X, growing on [13C]methanol, were supplemented with unlabelled amino acids. Uptake and participation in metabolism were determined via gas chromatography/mass spectrometry of derivatized amino acids, obtained from hydrolyzed cellular protein, by measuring their 12C content. Several amino acids appeared to be incorporated into the protein to a significant extent, without degradation or conversion. Among these were the aromatic amino acids, L-tyrosine and L-phenylalanine. Using the same replacement approach, their incorporation into PQQ was determined by 1H- and 13C-NMR spectroscopy of purified PQQ obtained from the culture medium. It appeared that the complete carbon skeleton of tyrosine was present, forming the o-quinone and pyrrole-2-carboxylic acid moieties in PQQ, while phenylalanine was not incorporated at all. Starting with L-tyrosine, possible biosynthetic routes to PQQ are discussed.