The influence of copper on the cycloheximide-induced synthesis of the copper-containing enzymes tyrosinase and laccase in Neurospora crassa was studied by enzyme activity measurements and immunological means. The amount of active enzyme molecules is far higher when the culture medium is copper-supplemented before cycloheximide induction. The synthesis of the apoproteins is not dependent on the presence of copper. This suggests the existence of a copper-storage protein for which metallothionein is a likely candidate.