SUMMARYThe drug-resistant variant, RSC-26, which was derived from the herpes simplex virus type 1 wild-type strain SC16, expresses an altered DNA polymerase and has reduced pathogenicity in animal models. To determine whether the attenuation in pathogenicity was due solely to mutation in the polymerase gene, a fragment of the wild-type gene was cloned, transferred into the genome of RSC-26 and recombinants were isolated. Three recombinants examined had similar properties to wild-type virus with respect to their sensitivity to antiviral drugs, DNA polymerase activities and their pathogenicity for mice. These results strongly suggest that expression of the altered polymerase of RSC-26 results in attenuated pathogenicity.