g-L-[3 H]-Glutaminyl-4-[14C]hydroxybenzene (doubly labelled GHB), a phenolic amino acid characteristic of the Agaricus genus, was synthesized by feeding direct precursors such as 4-[U-14C]aminophenol and L-[U-3 H]glutamic acid to mushroom gills. Radiolabelled compounds, namely mannitol, glutamine and doubly labelled GHB, were applied onto different tissues of growing sporophore and mycelium to follow their possible translocation into fruit body. Following contact with mushroom enzymes, doubly labelled GHB was immediately cleaved into its two components; the aromatic moiety was mainly consumed and lost as CO2. This result was confirmed by applying 4-[U-14C]aminophenol on both growing mushroom and sterile mycelium, which led to the recovery of the radiolabel as 14CO2.