In up to 45% of reported cases of the non-Hodgkin's lymphoma, diffuse large cell lymphoma, there are translocations of the BCL-6 gene, which are presumed to deregulate its expression. The BCL-6 protein, which is unmutated in these lymphomas, contains six Krüppel-like zinc fingers at its carboxy terminus and a 121 amino acid domain at its amino terminus, termed the POZ domain, which bears homology with amino terminal domains in a subset zinc finger transcription factors. In this study, we tested whether BCL-6 regulates transcription and if the POZ domain has a role in this function. The BCL-6 POZ domain, when fused to the GAL4 DNA binding domain, strongly repressed transcriptional activation initiated from several different promoters including the SV40 enhancer/promoter. Repression was also observed when the fusion protein was bound at a distance of 200 bp 5' of the promoter. When the GAL4/BCL6 POZ domain fusion protein was expressed in yeast, it was able to homodimerize in the nucleus. Nevertheless, in contrast with mammalian cells, the fusion protein did not repress transcription. To test the ability of the full length BC1-6 protein to repress transcription when bound to DNA through its zinc finger DNA binding domain, high affinity BCL-6 binding sites were selected from a pool of random oligonucleotides. Full length BCL-6 was able to strongly repress transcription when bound to its cognate site cloned upstream of the thymidine kinase promoter. This repression was mediated, in large measure, by the POZ domain, although a variant of BCL-6 lacking the POZ domain was able to repress transcription modestly. The ability of BCL-6 to function as a transcriptional repressor may contribute to its ability to transform B lymphocytes in diffuse large cell lymphoma.