Anaerobic treatment with cyanide of reduced ascorbate oxidase causes total depletion of copper. No significant amount of the metal is reincorporated when the apo-enzyme is incubated with cupric ions, but it is upon incubation with a stoichiometric amount (eight mol per mol of native enzyme) of a Cu(I) complex stable in air [Cu(I)(thiourea)3]Cl. The yield in reconstituted protein is higher under anaerobic conditions (85-90%) than in air (70-75%). By treatment with less than stoichiometric amounts of [Cu(I)(thiourea)3]Cl the apo-protein binds copper preferentially at the blue copper site. As a consequence the recovery of enzymatic activity is percentually lower than copper reincorporation.