A heat-labile protein has been identified in fruit bodies of the edible mushroom, Agaricus bisporus, which protects Raji cells (a human lymphoma cell line) against H2O2-induced oxidative damage to cellular DNA. This protein has been purified following salt fractionation, combined with ion-exchange, hydrophobic interaction and adsorption chromatography. Based on catalytic and electrophoretic properties, and inhibition studies using tropolone, the protein was identified as tyrosinase. The genoprotective effect of A. bisporus tyrosinase, determined using the single-cell gel electrophoresis met") assay, has been shown to be dependent upon the enzymic hydroxylation of tyrosine to L-DOPA and subsequent conversion of this metabolite to dopaquinone. The possible role of dopaquinone, and other L-DOPA oxidation products, in enhancing the cellular antioxidant defence mechanisms is discussed.