BCL-6 gene alterations have been observed in 27-45% of diffuse large B-cell lymphomas (DLBs) with chromosomal translocations at 3q27. The deregulated expression of normal BCL-6 protein caused by this chromosomal translocation is believed to be responsible for lymphomagenesis. Recently, we demonstrated that BCL-6 is expressed at high levels in germinal center B-cells as a 92-98 kDa nuclear protein in a constitutively phosphorylated form. In this study, we show that BCL-6 is phosphorylated by mitogen-activated protein kinase (MAPK) in vitro at the sites phosphorylated in vivo. These numerous phosphorylation sites were found to be located in its serine- and proline-clustered (SPC) region (amino acids-250-483). BCL-6 phosphorylation significantly increased in Ramos cells following stimulation with 12-o-tetradecanoylphorbol-13-acetate (TPA) or BCL-6- and erk1-transfected COS-7 cells stimulated with epidermal growth factor (EGF), and the increase of phosphorylation was inhibited by MEK1 inhibitor, PD98059. Furthermore, we observed that BCL-6 was associated with MAPK in vivo and its SPC region was important for this association. These results suggest that the functions of BCL-6 are regulated by phosphorylation mediated by the MAPK signaling pathway.