The chromosomal translocation involving 3q27 is often detected in human B-cell lymphomas, especially diffuse lymphomas with a large-cell component. The BCL6 gene has been isolated from the chromosomal breakpoint in these lymphomas. Here we cloned the murine BCL6 (mBCL6) cDNA from the muscle cDNA library using the human BCL6 (hBCL6) cDNA as a probe. The predicted amino acid sequence was 95% identical to that of hBCL6. It contains six repeats of the Krüppel-like zinc-finger motif that are completely identical to those of hBCL6, indicating that the BCL6 gene is well conserved between humans and mice. Expression of the mBCL6 gene was ubiquitously detected in adult mouse tissues including lymphatic organs. Furthermore, it was induced in lymphocytes activated with phorbol ester and Ca2+ ionophore within 30 min after stimulation. This induction was not inhibited by treatment of the cells with a protein synthesis inhibitor, cycloheximide. These results suggest that BCL6 plays a role in activated lymphocytes as an immediate early gene.