Interferon regulatory factors (IRFs) are transcription factors for interferon-related genes, which manifest both antiviral and tumor-suppressor activities and regulate cell growth in response to DNA damage. For the transcription initiation of the interferon-p gene, IRFs form a macromolecular assembly bound to the promoter DNA, referred to as an enhancesome, together with several other transcription factors and DNA-binding proteins. The three-dimensional structure of IRF-DNA complex would provide insights into the structure and function of the enhancesome, In this study, we crystallized the DNA-binding domain of interferon regulatory factor-a complexed with a DNA fragment. The crystals reproducibly grew by the vapor diffusion technique with 2-methyl-pentanediol from solutions containing small detergents, such as n-octyl-beta-D-glucoside. Cryocrystallographic experiments showed that crystals belong to space group P2(1)2(1)2(1) With a = 90.66 Angstrom b = 101.01 Angstrom, c = 171.58 Angstrom and diffract up to 2.8 Angstrom resolution. The absorption measurements of a solution in which the crystals were dissolved indicate that the DNA-binding domain binds to the DNA as a dimer. The calculated values of the solvent contents suggest that the protein-DNA complexes form a multimer in the crystal. These features may reflect the association of the complexes in the enhancesome.