The isolation, structure, and synthesis of agaritine (I), a unique phenylhydrazine derivative of L-glutamic acid from basidiomycetes of the family Agaricaceae, have recently been reported. As a prelude to studying the enzymic synthesis of this substance, relative levels of agaritine within successive sections of the stipe and fruiting cap of Agaricus bisporus were determined. Routine analyses were based on the observation that acidic solutions of β-N-acyl-phenylhydrazines very slowly form azo chromophores with N-1-naphthylethylenediamine (the coupling reagent in the Bratton-Marshall test for arylamines). However, extracts prepared from the basal portion of the stipe showed a coupling response whose rapidity was clearly more characteristic of a fully oxidized aryldiazonium ion than a hydrazide or hydrazine moiety. Evidence supporting the structure of this aromatic diazo derivative as p-hydroxymethylbenzene diazonium ion (II) included: (1) paper chromatographic Rf values of its arylazo derivatives (formed with N-1-naphthylethylenediamine and N,N-dimethylaniline) being indistinguishable from those of diazotized p-aminobenzyl alcohol; (2) hydrolysis of the basal-stalk extract with 0.25 N HCl at 100°C leading to almost complete disappearance of the azo-coupling response and formation of a phenolic derivative that migrated with the same Rf and hue as authentic p-hydroxybenzyl alcohol. Although the involvement of II in agaritine metabolism is currently speculative—equally likely arising from an enzymically catalyzed diazotization process or from chemical/enzymic oxidation of a related arylhydrazine—ongoing investigations in the laboratory on the mode of formation of the N-N bond of I should offer more direct information on this point.