A spliced cDNA spanning the Epstein-Barr virus BZLF1 gene expresses the BZLF1 protein and is active in inducing the virus productive cycle. A deletion mutant which lacks the N-terminal half of the protein is inactive. Cotransfection experiments in EBV-negative B-lymphocyte cell lines demonstrated that the BZLF1 gene activates the promoter for the BSLF2 + BMLF1 gene in the absence of any other EBV gene product. These results confirmed that the spliced BZLF1 gene is the transactivating gene structure in BamHI-Z.