Cellular and viral Fos proteins form a tight complex with other nuclear proteins, including the transcription factor and proto-oncogene AP-1/Jun. We have mapped the c-Jun binding site in Fos to a region containing regularly spaced leucine residues recently suggested to interdigitate with a similar structure in Jun. Substitution of single or multiple leucine residues or the alteration of leucine phasing by insertion of additional amino acids reduces or abolishes the binding to Jun, while the substitution of other amino acids has no noticeable effect. These results strongly suggest that the formation of a "leucine zipper" mediates the interaction between Fos and Jun. We also show that the differential binding of the various Fos mutants correlates with their potential to trans-activate AP-1-dependent transcription and to induce morphological transformation.