IT was shown that laccase, which had been inactivated by cyanide under certain conditions, could be almost completely re-activated by cupric ions, whereas other metals were ineffective1. Further evidence will be presented here to show that copper is an essential constituent of this enzyme. Laccase was purified by the method of D. Bertrand2,3. At each successive stage of purification the enzyme activity was measured in the manner previously described1, and copper and manganese were estimated by microchemical methods4,5. Manganese disappears after the first steps of purification; the copper content increases with the and remains proportional to activity in all fractions up to the purest preparation having 30,000 in presence of paraphenylene diamine (Fig. 1).