Induction and activation of the p53 tumour suppressor protein occurs in response to a number of cellular stresses, including disruption of RNA polymerase II-mediated transcription. Both p53 itself and its principle negative regulator, the E3 ubiquitin ligase Mdm2, are substrates for phosphorylation by the protein kinase CK2 in vitro. CK2 phosphorylates Mdm2 within its central acidic domain, a region that is critical for making a second point of contact with p53 and mediating p53 ubiquitylation and turnover. Additionally, there is evidence that CK2 interacts with, and regulates, both p53 and Mdm2 within the cell but the molecular mechanisms through which CK2-mediated regulation of the p53 response can occur are only poorly understood. Previously, we showed that the basal transcription factor TAFII250, a critical component of TFIID, can interact with Mdm2 and promote the association of the Mdm2 acidic domain with p53. In the present study, we show that immunoprecipitates of TAFII250, either from mammalian cell extracts or from baculovirus-infected cells expressing elevated levels of HA-tagged TAFII250, can phosphorylate Mdm2 in vitro within its acidic domain. We show that CK2 is tightly associated with TAFII250 and is the principal activity responsible for TAFII250-mediated phosphorylation of Mdm2. Our data fit with recent observations that phosphorylation of the acidic domain of Mdm2 stimulates its association with p53 and are consistent with a model in which recruitment of CK2 by TAFII250 may play a contributory role in the maintenance of Mdm2 phosphorylation and function.