The effect of L-cysteine on mushroom polyphenol oxidase (PPO) activity was investigated using three spectrophotometric assays. The formation of pigment (melanin), o-phenylquinone and cysteine-quinone adduct from catechol were each assayed under similar conditions, Cysteine had two effects; Jirst, a lag phase was seen when melanin formation was measured, and secondly, the rate of browning was decreased after the lag phase. The lag phase was not observed when the formation of cysteine-quinone adduct rather than melanin formation was measured. This suggests that the lag phase observed in melanin formation is due to adduct formation and not quinone reduction. The velocity of adduct formation was similar to the velocity of o-phenylquinone and melanin formation without cysteine suggesting that reduction of o-phenylquinone to catechol was not sign$cant. The structure of the adduct formed between cysteine and ophenylquinone was synthesized and unequivocally determined by NMR spectroscopy to be S-(2,3-dihydroxyphenyl)cysteine.