A method for restoring superconductivity of a wire having a core of reacted MgB.sub.2 comprises subjecting said wire to a two-phase heat treatment, wherein a first phase comprises heating in a range of 800 C. to 1000 C. during 20 minutes to 40 minutes and a second phase comprises heating in a range of 550 C. to 750 C. during 45 minutes to 75 minutes.