1,2-Naphthoquinone (1,2-NQ) is produced during incomplete combustion of gasoline and photooxidation of naphthalene in the atmosphere. While 1,2-NQ is found to covalently bind protein nucleophiles in cells, the extracellular regulation is not understood. In the present study, we found that 1,2-NQ is readily trapped by cysteine released from A431 cells, leading to the cysteine adduct formation, thereby repressing covalent modification of the cellular proteins. We also identified such a cysteine adduct as 1,2-dihydroxynaphtalene-cysteine adduct by LC/MS analysis with the synthetic chemical.