The transcriptional repressor B lymphocyte-induced maturation protein-1 (Blimp-1) is expressed in some differentiated cells and is required for terminal differentiation of B cells. To facilitate identification of Blimp-1 target genes, we have determined the optimal DNA recognition sequence for Blimp-1. The consensus is very similar to a subset of sites recognized by IFN regulatory factors (IRFs) that contain the sequence GAAAG. By binding competition and determination of equilibrium dissociation constants, we show that Blimp-1, IRF-1, and IRF-2 have similar binding affinities for functionally important regulatory sites containing this sequence. However, Blimp-1 does not bind to all IRF sites, and specifically does not recognize IRF-4/PU. 1 or IRF-8 sites lacking the GAAAG sequence. Chromatin immunoprecipitation studies showed that Blimp-1, IRF-1, and IRF-2 all bind the IFN-β promoter in vivo, as predicted by the in vitro binding parameters, and in cotransfections Blimp-1 inhibits IRF-1-dependent activation of the IFN-β promoter. Thus, our data suggest that Blimp-1 competes in vivo with a subset of IRF proteins and help predict the sites and IRF family members that may be affected.