In this paper we propose to describe a comparatively simple method of preparation and purification of a polyphenol oxidase. The study of this highly purified enzyme makes possible the determination of its active group and of its main properties which are m asked in crude preparations even when they are highly active. Material. The best source of this enzyme is presented by cultivated mushrooms which belong to one of the variety of Agaricus or Psalliota campestris somewhat modified by continual and intense cultivation. This material has the advantage of being very rich in enzyme and of being available in large quantities practically all the year round.