A region of the Epstein-Barr virus (EBV) genome that is important in inducing cell proliferation includes a single long open reading frame. Part of this open reading frame has been fused to the lacZ gene and expressed in Escherichia coli. Antisera to the fusion protein identify a protein in the nuclei of latently infected growth-transformed lymphocytes and in Burkitt tumor cells grown in vitro. This nuclear protein is encoded by a different virus-gene than that which encodes the previously described EBV nuclear antigen, EBNA.