The 'cysteine-zinc DNA binding finger' is a recently identified sequence motif that is present in a wide variety of transcriptional regulatory proteins and is thought to be directly involved in DNA binding. It has been proposed that an essential component of this structure is a zinc ion bound between two pairs of cysteine residues. The GAL4-encoded protein of Saccharomyces cerevisiae, which binds to DNA and activates the transcription of several genes, contains this sequence motif. Here I describe a gal4 mutant with an alteration in the cysteine-zinc DNA binding finger whose defect is corrected in vivo by high concentrations of ZnCl2. The DNA binding activity of the altered protein from this mutant is restored by ZnCl2 in vitro. This is evidence that the GAL4 protein indeed contains zinc ions essential for its DNA binding activity.