The induction of a copper-binding metallothionein (Cu-MT) was studied in yeast, Saccharomyces cerevisiae, and a relationship between copper resistance and intracellular levels of Cu-MT in these eukaryotes was established. Poly(A)-containing RNA from a copper-resistant (Cur) yeast strain, which synthesized abundant quantities of Cu-MT and in which Cu-MT gene transcription was enhanced 50-fold upon exposure to CuSO4, was used to screen yeast genomic DNA clones. Restriction analysis revealed common XbaI and KpnI sites in five genomic clones isolated. The transcription of these clones was regulated by copper. Transformation of a copper-sensitive (Cus) yeast strain by one of these clones confers copper resistance in yeast. The results suggest that the expression of the Cu-MT gene is, in part, responsible for mediating copper resistance in yeast.