Three independently isolated herpes simplex virus type 1 recombinant viruses containing a deletion of approximately 14 kilobase pairs, representing greater than 95% of the internal repeat DNA sequences, were analyzed for their pathogenicity in mice. The recombinant viruses were found to be avirulent, exhibiting drastically increased LD50 values over wild-type herpes simplex virus 1(F) by intracerebral injection, nonneuroinvasive, unable to spread from the cornea to sensory ganglion, and unable to establish a reactivable latent infection in trigeminal ganglion following either intracerebral inoculation or inoculation of scarified corneas. The potential role of diploid genes in herpes simplex virus pathogenesis in the mouse is discussed.