A replication-defective mutant of herpes simplex virus 2 (HSV-2) was engineered by replacing the ICP8 gene of HSV-2 strain 186 with an ICP8–lacZ fusion gene from the herpes simplex virus 1 (HSV-1) HD-2 mutant strain. The resulting virus, HSV-2 5BlacZ, is defective for growth in Vero cells but is capable of growth in a cell line that expresses HSV-1 ICP8. In Vero cells, the mutant virus is defective for DNA synthesis but is able to express many viral proteins at levels similar to those of wild-type virus, including several of the late kinetic class. SDS–PAGE and Western blot analysis demonstrated the expression of glycoproteins B and D by 5BlacZ in Vero cells. Initial studies have shown that immunization with 5BlacZ protects guinea pigs from intravaginal HSV-2 challenge. Immunized animals had less severe genital skin disease and reduced replication of the challenge virus in the genital tract during primary infection and reduced episodes of recurrent disease. Thus, HSV-2 ICP8 shows gene regulatory properties similar to those of HSV-1 ICP8, and this HSV-2 ICP8 mutant virus shows a phenotype similar to those of HSV-1 ICP8 mutant strains. Replication-defective mutants of HSV-2 offer a potential vaccine approach for immune intervention against HSV-2 genital disease and latent infection.