Virtually all diffuse large cell lymphomas and a significant fraction of follicular lymphomas contain translocations and/or point mutations in the 5' non-coding region of the putative oncogene BCL-6, that are presumed to deregulate its expression. BCL-6 encodes a Cys2-His2 zinc finger transcriptional repressor with a POZ domain at its amino-terminus. The POZ (or BTB) domain, a 120-amino-acid motif, mediates homomeric and, in some proteins, heteromeric POZ-POZ interactions. In addition, the POZ domain is required for transcriptional repression of several proteins, including BCL-6. Using a yeast two-hybrid screen, we identified N-CoR and SMRT as BCL-6 interacting proteins. Both N-CoR and SMRT, which were originally identified as co-repressors for the unliganded nuclear thyroid hormone and retinoic acid receptors, are components of large complexes containing histone deacetylases. We show that the interaction between BCL-6 and these co-repressors is also detected in the more physiologically relevant mammalian two-hybrid assay. The POZ domain is necessary and sufficient for interaction with these co-repressors. BCL-6 and N-CoR co-localize to punctate regions of the nucleus. Furthermore, when BCL-6 is bound to its consensus recognition sequence in vivo, it can interact with N-CoR and SMRT. We find, in vitro, that POZ domains from a variety of other POZ domain-containing proteins, including the transcriptional repressor PLZF, as well as ZID, GAGA and a vaccinia virus protein, SalF17R, also interact with varying affinities with N-CoR and SMRT. We find that BCL-6 POZ domain mutations that disrupt the interaction with N-CoR and SMRT no longer repress transcription. In addition, these mutations no longer self associate suggesting that self interaction is required for interaction with the co-repressors and for repression. More recently N-CoR has also been implicated in transcriptional repression by the Mad/Mxi proteins. Our demonstration that N-CoR and SMRT interact with the POZ domain containing proteins indicates that these co-repressors are likely involved in the mediation of repression by multiple classes of repressors and may explain, in part, how POZ domain containing repressors mediate transcriptional repression.