DNA fragments containing an open reading frame known to encode most or all of the EBNA1 protein of Epstein-Barr virus (EBV) were fused in the proper transcriptional orientation to the promoter regulatory domain, capping site, and a portion of the 5' transcribed noncoding sequences of the HSV-1 alpha 4 gene of herpes simplex virus 1 (HSV-1). In these constructs 20, 130, or 385 bp of EBV DNA and 28 bp of HSV-1 DNA separated the alpha 4 cap site from a putative initiator codon of the EBNA1 gene. The chimeric alpha 4-EBNA1 genes were introduced into L cells or recombined into the viral genome using the thymidine kinase selection system. The three chimeric gene constructs resident in the L cell clones expressed a protein indistinguishable from authentic EBNA1 with respect to electrophoretic and immunologic properties indicating that the ATG at the beginning of the EBV open reading frame initiated translation of the bonafide EBNA1 protein. The chimeric alpha 4-EBNA1 genes resident in L cells were induced by HSV-1 infection and were regulated as alpha genes. The chimeric alpha 4-EBNA1 gene recombined into the viral genome was also regulated as an alpha gene. The recombinant viruses were stable and expressed 50- to 100-fold more EBNA1 than is ordinarily expressed in human lymphocytes carrying the EBV genome. EBNA1 did not alter the program of HSV-1 protein expression. The utility of the vector is discussed.