WE have previously shown1 that laccase, the oxidizing enzyme present in the latex of the Indo-Chinese lacquer tree (Rhus succedanea), is a copper-protein compound. Our purest preparation of this enzyme, which catalyses the oxidation of polyphenols and diamines, contained 0·154 per cent copper and 6·45 per cent nitrogen, corresponding to about 45 per cent protein. The remaining fraction of the preparation was composed of a polysaccharide. This preparation had a strong blue colour which disappeared reversibly on the addition of reducing substances or substrates and irreversibly on heating the enzyme to about 60° C. or acidifying it.