The native amino acid ergothioneine, a thiourea derivative of histidine, inhibits mushroom tyrosinase activity in a dose-dependent manner, with an IC50 value of 1.025 mg/ml (4.47 mM). By contrast, histidine exhibited no inhibitory effect on mushroom tyrosinase activity. We characterized ergothioneine as a noncompetitive tyrosinase inhibitor using a Lineweaver-Burk plot of experimental kinetic data. The IC50 value for ergothioneine scavenging of 2,2-diphenyl-1-picrylhydrazyl was 6.110+/-0.305 mg/ml, much higher than the IC50 for inhibition of tyrosinase activity which indicating ergothioneine on tyrosinase shows a weak correlation to its antioxidative activity. The results demonstrated that ergothioneine has a potent inhibition effect on tyrosinase enzyme activity, resulting from the presence of the sulfur substituted imidazole ring in ergothioneine.