The EBNA1 protein of Epstein-Barr virus (EBV) activates DNA replication by binding to multiple copies of its 18-bp recognition sequence present in the Epstein-Barr virus latent origin of DNA replication, oriP. Using electrophoretic mobility shift assays, we have localized the minimal DNA binding domain of EBNA1 to between amino acids 470 and 607. We have also demonstrated that EBNA1 assembles cooperatively on the dyad symmetry subelement of oriP and that this cooperative interaction is mediated by residues within the minimal DNA binding and dimerization domain of EBNA1.