Magnesium diboride, recently discovered as superconductor, is extremely interesting in respect to pairing mechanisms and phonon-mediated superconductivity, but its transition temperature (39K) is decidedly too low for practical applications. A rush for substitutions started, intended to raise Tc and possibly maintain acceptable critical current density. The difficult introduction of Cu was attempted, through a complex procedure employing CuB24 instead of metallic Cu: owing to the tendency of Cu to form inter-metallic compounds with Mg, only a maximum of 3mol% of Cu was safely substituted to Mg in MgB2 lattice and a small parameters contraction proportional to Cu content was recorded. The results are a decrease in Tc of about 0.5 K, not far from that found with Al addition, whose reasons are discussed and interpreted in terms of holes doping.