Most of the studies about UV irradiation of fruit and vegetable derivatives have been carried out in order to assess its effect on microbial inactivation. Nevertheless, there are few references about UV influence on some enzyme activities that are important in this kind of food, especially polyphenol oxidase, which is responsible for enzymatic browning in fruit and vegetable tissues containing phenolic or polyphenolic compounds. In this work, the effect of UV-Vis irradiation on polyphenol oxidase from Agaricus bisporus was investigated. A reduction of 58.7% in enzyme activity was achieved in the first 90 s, and it was completely inactivated after 35 min of treatment. In addition, the protective effect of melanins synthesized by the action of the same polyphenol oxidase was assessed. These pigments absorbed some of the radiant energy and led to a slower inactivation of polyphenol oxidase. (C) 2011 Elsevier Ltd. All rights reserved.