Several regulatory interactions between the AP-1 and the nuclear hormone receptor families of transcription factors have been reported. However, the molecular mechanisms that underlie these interactions remain unknown, and models derived from transient-transfection experiments are contradictory. We have investigated the effect of the purified glucocorticoid receptor (GR) DNA-binding domain (GR residues 440 to 533 [GR440-533]) on DNA binding and transcription activation by Fos-Jun heterodimers and Jun homodimers. GR440-533 differentially inhibited DNA binding and transcription activation by Fos-Jun heterodimers. Inhibition of Jun homodimers required a 10-fold-higher concentration of GR440-533. An excess of Fos monomers protected Fos-Jun heterodimers from inhibition by GR440-533. Surprisingly, regions outside the leucine zipper and basic region were required for GR inhibition of Fos and Jun DNA binding. The region of GR440-533 required for inhibition of Fos-Jun DNA binding was localized to the zinc finger DNA-binding domain. However, inhibition of Fos-Jun DNA binding was independent of DNA binding by GR440-533. GR440-533 also differentially inhibited Fos-Jun heterodimer binding to the proliferin plfG element. Differential inhibition of DNA binding by different AP-1 family complexes provides a potential mechanism for the diverse interactions between nuclear hormone receptors and AP-1 family proteins at different promoters and in different cell types.