PUA and IRF-4, members of the Ets and interferon regulatory families of transcription factors, respectively, form a cooperative ternary complex that is implicated in the regulation of B-cell-specific gene expression. A portion of the cooperativity involves interaction between the PUA and IRF-4 DNA-binding domains. We report here the crystallization and preliminary characterization of PU.1 and IRF-4 DNA-binding domains bound to a 21-mer DNA site from the lambdaB element of immunoglobulin light chain lambda(2-4) enhancer. The crystals belong to space group P2(1) with unit cell dimensions of a=40.7 Angstrom, b=62.3 Angstrom, c=67.9 Angstrom, beta=95.6degrees with one complex molecule per asymmetric unit. Crystals diffract to a resolution of 3 Angstrom using X-rays from a rotating anode generator but improve to 2.3 Angstrom with synchrotron radiation. The crystals are highly mosaic, but the mosaicity can be improved following a series of steps involving the addition of additives, use of peritone oil as a cryoprotectant, slow flash-cooling in the cryostream, and several cycles of crystal annealing. (C) 2002 Elsevier Science (USA). All rights reserved.