Recently, a hypothetical structure called a leucine zipper was proposed that defines a new class of DNA binding proteins. The common feature of these proteins is a region spanning approximately 30 amino acids that contains a periodic repeat of leucines every seven residues. A peptide corresponding to the leucine zipper region of the yeast transcriptional activator GCN4 was synthesized and characterized. This peptide associates in the micromolar concentration range to form a very stable dimer of alpha helices with a parallel orientation. Although some features of the leucine zipper model are supported by our experimental data, the peptide has the characteristics of a coiled coil.