1. Conditions for preparation of apolaccase EC 1.10. 3. 2 and reconstruction of the holo-enzyme by treatment with copper ion were examined. The apo enzyme was prepared by dialysis against 10–20 mM cyanide solution containing 20–50 mM ascorbate at pH 7.0–7.4. The properties of native laccase, such as its activity, copper content, absorption spectrum and electron spin resonance spectrum, were almost completely regained by treatment of this apo-protein with cuprous ion and the percentage reconstruction was 70 to 90%. Some, though much less reconstruction was achieved by treatment of apo-enzyme with cupric ion. 2. The relationships between the copper content, the absorbancy at 615 mμ and the activity were studied using “partial apo-enzymes,” derivatives of native enzyme containing various amounts of copper obtained during cyanide-treatment. The decrease in activity of the “partial apo-enzymes” was much larger than the decrease in their copper content. Thus, removal of half the initial amount of copper resulted in loss of more than 90% of the original activity. The cooperative action of copper atoms in the laccase reaction is suggested.