Agaritine [(β-N-(γ-L(+)-glutamyl)-4-hydroxymethylphenylhydrazine] was isolated and purified from the mushroom Agaricus bisporus. The prevention of melanin formation by agaritine is reported here for the first time. This compound, characteristic for the Agaricus genus, was a nucleophile in its deprotonated state which was able to remove the, either enzymatically or chemically generated, o-quinones from the assay medium. The disappearance of these o-quinones followed first order kinetics through a decreasing uniexponential. The dependencies of the apparent rate constant (k(q)) for the disappearance of the o-quinone, as well as the o-quinone concentration removed from the assay medium ([Q]0) were studied as a function of proton, agaritine and o-quinone initial concentrations. k(q) increased linearly with agaritine concentration, decreased non-linearly with proton concentration and was independent of the initial o-quinone concentration. [Q]0 showed a linear increase with initial o-quinone concentration, a non-linear decrease with proton concentration and was independent of the initial agaritine concentration. Taking into account the effect of agaritine on o-quinones, this paper calls for caution in the use of agaritine in experiments with o- quinone accumulation. Moreover, a possible role for agaritine in the browning of mushrooms is discussed.